Speedometer recalibration: is it possible?

[Grant Eaton]

If I get larger wheels for my VX, can the speedometer be recalibrated fairly easily? Perhaps by the dealer?

[AlaskaVX]

Why would you want to do a thing like that? I can see doing it if you had smaller tires on there, but with larger tires your actually putting less miles on your car. They do have things that show true speed. Just remember what the differance is to a % eventually you won't even have to think about it you just know that 50 mph = 55mph and 60 =66 or however your size tires work out compared to stock (29" or 29.5"). Point being don't spend money you don't need to.

[coachreed]

I considered asking the same question... I found by my speeding ticket that I got on the way to Florida a couple weeks ago that my speedo is about 8% off... that is with my 285/60/18's. Then I thought the same as AlaskaVX... why add more miles on than I need to... I'll get used to the speedo.

TTYL!

Coach... aka Randy!

[Grant Eaton]

Originally posted by AlaskaVX
Why would you want to do a thing like that? I can see doing it if you had smaller tires on there, but with larger tires your actually putting less miles on your car.

I have this thing about instruments (and things in general) being precise. It must be the German in me. :rolleyes:

I don't care about mileage; I'll own this truck as long as I'm able to drive.

[WyrreJ]

I think you can buy a box that sits in between your speedo and your wheels that will do the conversion for you.

But, you could also use it as an excuse to get yourself a GPS unit and just use it to tell you what your true mph is. Often speedos are undercalibrated at the factory, but a good GPS will give you a pretty precise number. Or so I have been told.

[Heraclid]

Also, about the 8% off... radar has been known to clock houses at 12 mph. And I don't mean a mobile home. :-)

Not saying it was what happened with the ticket you got, but police radar is notoriously inaccurate, depending on various factors.

[Tone]

Autotrans offer a recalibrator and I currently have one for testing - is there sufficient interest for this device? It costs approximately $150 and must be spliced into the harness under the vehicle.

[Swordy]

If you want to do the job right, you change your rear end gears to compensate for the change in torque due to the tire size. On some tranny's there are speedometer gears that you can access from under the car for a quick fix. I don't believe ours have that though. Other than that, it's the black box thing tones talking about.

[Grant Eaton]

I ended up buying a set of Pirelli Scorpion Zeros from www.tirerack.com. I paid about $154 each plus $70 shipping (total cost was about $700).

I bought the 285/55/R18 size. At 30.34", the diameter of the tire is about 2.5% larger than stock, so the speedometer will be a little off, but not too much.

The other wide-tire option, 285/50/R18, was about 1.1% smaller than stock at 29.22" diameter. (Stock is 29.57".) I figured I would rather make an extra 2.5% on my business mileage expense reports with the bigger tires. :naughty: Plus, the 285/55/R18's were $40 cheaper for the set than the 285/50/R18's.

I don't want to do any cladding trimming and from everything I read, this size tire should fit without rubbing the cladding or anywhere else.

(I hope.) :right:

[Grant Eaton]

Originally posted by Tone
Autotrans offer a recalibrator and I currently have one for testing - is there sufficient interest for this device? It costs approximately $150 and must be spliced into the harness under the vehicle.

http://www.autotransinc.com/recal.jpg

The Web site is here:

http://www.autotransinc.com/Recal.htm

Actually, I read up on this and I am a little interested. According to their Web site, the ABS system uses speed calculations to determine proper behavior. Although it isn't mentioned, I imagine the speed calculations also affect airbag deployment parameters. My new tires will alter the VX's speed calculation by 2.5% -- that's probably pretty significant when you're talking about the delicate matrix of conditions required to trigger airbag deployment. I wouldn't want to take a risk with impairing the bag's trigger characteristics.

So... let me know how your testing goes and I might pick one up from you if they aren't too difficult to install or have an unadvertised trade-off. ;)

[Tone]

I’ll try to nail something down this week. Actually your speedo is going to UNDER report both distance and speed so you’re actually loosing money on the expense report but the good side is the odo will not age as quickly! ;)

[johnnyapollo]

A good GPS will give you actual speed, so you could mount one and ignore the speedometer readings. Just another option.

-- John

[coachreed]

Originally posted by Heraclid
Also, about the 8% off... radar has been known to clock houses at 12 mph. And I don't mean a mobile home. :-)

Not saying it was what happened with the ticket you got, but police radar is notoriously inaccurate, depending on various factors.

Yeah, I know radar has been known to be inaccurate at times, but the number I got from my ticket pretty much went with my own figures... paceing and timeing mile markers. I had already driven about 400-500 miles that day and figured I was off by around 7%-8%. Then after my ticket was issued, I asked the cop to pace me at 65 (the speed limit) and I was reading 59-60mph. Then later in my trip, I had my friends grandfather pace me on the way back up to GA from FL... and his was reading about the same... so, my conclusion is that I'm off about what I figured... 8% give or take.

Grant-
It looks to me like your figuring a percentage error with the diameter, right? You need to do an error calculation with the circumference as this is the actual distance traveled per revolution.

Don't know if I care to adjust mine though. I'll take every bit of extra warranty I can get. I am currently working on a complete auto-pc system that will include GPS so I'll have an accurate number then...

TTYL!

Coach... aka Randy!

[IsuZOOM]

Originally posted by Heraclid
police radar is notoriously inaccurate Our site owner might take issue with that :D

I know with my boat, (and many boats) GPS is WAY more accurate than the speedometer in the dash. I imagine the same is true with automobiles. I recall an article in a car magazine a short while back where they compared actual speed (tested with what I think they call a fifth wheel) versus reported speed on the speedometer and every car was off by at least 2 MPH, some as much as 10 at high speeds!

[Moncha]

Because of discrepancies, our local policy is a 10mph given. Anything over, zero tolerance.

[WyrreJ]

Diameter and circumference are directly proportional, so a 2% increase in diameter will result in a 2% increase in circumference.

From high school geometry class: circumference = π * diameter

[Moncha]

Remember to use the good ol' tire calculator (http://www.miata.net/garage/tirecalc.html), makes life much more simple. It gives you a % +/- of speedo difference.

[Grant Eaton]

Originally posted by coachreed
Grant-
It looks to me like your figuring a percentage error with the diameter, right? You need to do an error calculation with the circumference as this is the actual distance traveled per revolution.


You're right. Let's see, isn't the equation pi * D?

So, stock = 3.141592 * 29.57 = 92.9"

My new Pirelli tires = 3.141592 * 30.34 = 95.3"

95.3" / 92.9" * 100 - 100 = -2.52%

So when my odo reads 65 MPH, I'll really be going about 66.6 MPH.

No one else has commented about the fact that having an inaccurate speedometer changes the behavior of your airbag deployment and possibly ABS. If your big tires are 8% off, that's a pretty significant piece of false data to be feeding your life-safety system. ;eeky;

[mrtew]

Originally posted by Grant Eaton
So when my odo reads 65 MPH, I'll really be going about 66.6 MPH.

No one else has commented about the fact that having an inaccurate speedometer changes the behavior of your airbag deployment and possibly ABS. If your big tires are 8% off, that's a pretty significant piece of false data to be feeding your life-safety system. ;eeky;

Another thing that no one has mentioned is that speedos aren't all that accurate from the factory anyway. One of the big car magazines always used to test their accuracy as part of the road test and they almost never were right. The were also almost always reporting the speed too high (presumably to keep the drivers 'out of trouble') so by putting bigger tires on your truck you very well might have made your 'instrument' MORE precise. In fact maybe you should have gone for the '60's. ;-]

As far as the airbag, I think the line beween it going off and not going off is around 15 miles per hour isn't it? At that speed 8% is only 1 or two miles per hour so I think I can take that risk. Besides I think that they measure your speed of collision by measuring the force of the decelleration with their sensors, not by tapping into the speedo.

[paultvx]

I can see how it can be a big deal with ABS functions... but with airbag depolyment?

Airbag deployment is the end result of tripping triggers that are placed in strategic locations within the vehicle. These triggers are usually some kind of magnetic switches where the circuit is completed if the force of impact is great enough to "flip" the switch. In most systems the airbag would only deploy if more than one switch has been triggered due to impact.

At 2.2% difference I don't see how the effects on the ABS system would matter much. If you are triggering the ABS left and right then you're breaking way too late and way too hard... and if your airbags have deployed, you've got other things to worry about besides that 2.2% difference.

What this all boils down to is that when you make alterations, you have to expect that your vehicle has been alterred in more ways than one... and you have to adjust your driving accordingly. As long as you are subject to the laws of gravity and physics, no amount of mods will make a 4k lb VX with high center of mass handle like a Porsche 911.

[WyrreJ]

Originally posted by mrtew
Another thing that no one has mentioned is that speedos aren't all that accurate from the factory anyway. Am I invisible in this thread or what? Three times so far, how bizarre...

[paultvx]

Originally posted by WyrreJ
Am I invisible in this thread or what? Three times so far, how bizarre...

Did someone say something? :D

[AnalogVX]

You guys are putting WAY too much thought into installing bigger tires. :p

[sveltax]

Isn't one other issue the fact that speedometers are not accurate from the f... enough of that!

What about resale and inaccurate speedometer readings? Is this an issue with a dealer or a private party? Does it need to be disclosed upon selling an altered vehicle?

I don't see a dealer saying "how long have you had your tires... really, well I think your warranty is up due to the 8.8% error factor due to the larger tire diameter.." but I guess it's possible.

[paultvx]

Originally posted by sveltax
Isn't one other issue the fact that speedometers are not accurate from the f... enough of that!

What about resale and inaccurate speedometer readings? Is this an issue with a dealer or a private party? Does it need to be disclosed upon selling an altered vehicle?

I don't see a dealer saying "how long have you had your tires... really, well I think your warranty is up due to the 8.8% error factor due to the larger tire diameter.." but I guess it's possible.

The dealer doesn't care. In fact, they'd say that the vehicle has been modified and use it as an excuse to low ball your trade-in value. I know this because I've experienced it. The private part doesn't care either. If the private party isn't looking for a modified vehicle then they simply won't buy it. If they don't mind buying a modified vehicle, well... the more you have the sweeter the deal... either way... it doesn't matter.

With regards to warranty, well some dealers will come up with any excuse to void warranty or refuse warranty claims. We already know that to be a fact... and not limited Isuzu and its dealers.

[mrtew]

Originally posted by paultvx
With regards to warranty, well some dealers will come up with any excuse to void warranty or refuse warranty claims. We already know that to be a fact... and not limited Isuzu and its dealers.


Why the heck would a dealer want to void or refuse a warranty claim. They still get paid by the Manufactutor for their work don't they? My dealer seems to LOVE to do it, especially when there's an error in his book and he gets paid for 1.5 hours of work for snapping out the popped up dash vent and popping in a new one.

[paultvx]

Originally posted by mrtew
Why the heck would a dealer want to void or refuse a warranty claim. They still get paid by the Manufactutor for their work don't they? My dealer seems to LOVE to do it, especially when there's an error in his book and he gets paid for 1.5 hours of work for snapping out the popped up dash vent and popping in a new one.

Some people don't live in Kansas, Dorthy. Haven't you read about all the problems people have had with cracked hood inserts? and how much pain it was for most of them to get replacements covered under warranty? and how many replacement inserts have cracked? :rolleyes: And that's just the hood insert. There's the chrome bubbling off of wheels... there's window regulators... there's UV damage or whatever it is on the headlight lenses... there's differential whine... there's driveline knock... there's headlight gasket falling off... Anyone else care to add to this list? Just because everything is peachy at one dealer doesn't mean it's that way for the rest of the world.

[mrtew]

Toto, I wasn't saying everything is peachy at my dealer, and I don't think it is at all, I was actually asking, WHY would a dealer refuse warrantee service? Does it cost them money? I was always under the impression that they got paid either way. And yes I've read about those problems, but most of what I've read is, "I just got new headlites and new $2000 wheels for free just because the rubber came unglued and the chrome had some bad spots on the inside where you can't see it." I wasn't trying to start a fight or make everyone jealous... I just wanted to know WHY they would refuse warrantee claims. Maybe if we understand that we can figure out how to solve the problem.

[paultvx]

They get paid when Isuzu OK's the repair. It's kind of like how some insurance companies operate. Insruance claim... Warranty claim. If the dealer goes ahead and does a repair and end up with Isuzu saying "NO" after all is said and done, the dealer looses 1) time, in preparing the paper work 2) money, time is money and they have to pay the mechanics to do the work plus the time they spent to work on your VX, they could've spent it on a paying, non-warranty, cash-in-hand repair.

That's why some dealers are very careful/strict about what can be considered as a warranty repair. It's their a** they're looking out for whenever "warranty" is mentioned. The dealer's bottomline as a business is to make money (what business wouldn't?). Warranty issues can be risky for them to take on from a business point of view. Think of it this way... you already have a new truck. Do they care if they piss you off? No, because they know you're not likely to make another new purchase anytime soon... therefore, no incentive for them to make sure you're 100% happy. The money they make on service is peanuts compared to what they make with each new car/truck sold. The situation is worse now because of Isuzu's financial problems. In many cities only one Isuzu dealer remains. In those situations the owners are at the mercy of that lone dealer. Supply and demand. One supplier... lots of demand. The supplier can afford to dictate the terms.

My dealer for example, makes a point not to stock parts (aside from the usual oil filter and belts) just so they can delay warranty repairs in order to buy time to get Isuzu's OK firsthand... and even then it takes a lot of convincing just to get them to entertain the idea of having something covered under warranty. It all depends on whether they are favored by the manufacturer or not. Most dealer don't want to deal with warranty repairs simply because there isn't enough incentive for them to go through the trouble. They'd rather sell cars and make the quick bucks. Unless you're a big shot newspaper editor who can print a full page ad saying "don't buy your cars here" there's no reason for the dealer to go out of their way for you. I mean to them we're just average Joes. If we don't come back, they can always find another sucker. Most dealers also sell more than just Isuzu. They could always say screw Isuzu and drop the whole line and would still do just fine selling the other makes they have. There's a thread here or on Vmag regarding Don-A-Vee in SoCal. They were one of the biggest Isuzu dealers around and they just dropped the brand. Considering Isuzu's uncertain future, your satisfaction is pretty irrelevant to dealers like that. The survival of their business doesn't depend on the opnion of few... and that's who Isuzu owners are nowadays... the few. It shouldn't be that way, but that's the way it is.

[mrtew]

Originally posted by paultvx
...because they know you're not likely to make another new purchase anytime soon... therefore, no incentive for them to make sure you're 100% happy. The money they make on service is peanuts compared to what they make with each new car/truck sold.... It shouldn't be that way, but that's the way it is.


Thanks for the thoughtful and detailed answer. Too bad they can't see that their treatment of customers DOES affect their sales. I've heard people complaining about my Isuzu dealer (Nissan customers mostly, which they also sell) long before I bought my truck and almost didn't get one because of it.

I think I disagree with your statement that they make more money selling cars than servicing them though. I have read several times in the past that many new car showrooms are money losers and are only there to support the Used car lots and Service centers that they are affiliated with, where all the money is made. Not true?

[paultvx]

The situation may vary from dealer to dealer. Dealers in economic depressed areas are definitly not going to make their living selling new cars, that's for sure.

In general, I think the dealers makes a couple to a few thousand on the new cars they sell. Sometimes there are factory incentives that don't get passed down to the buyer. So, suppose they make 2 grand per car and within a 8 hour business day they sell 4 cars, that's 8 grand. On the service side, suppose on average they charge $80 for regular service and repairs per job/vehicle. How many cars would they have to service in a day to equal what they make by selling 4 cars?

Like I said... it's probably different from dealer to dealer. Some are service oriented (and have a lot of service bays)... some are more sales oriented. Depends on what their business model is.

[WyrreJ]

Gross yeah, but to net $2K per vehicle they are either selling luxo brands or something with enough hype that people will pay stupid money for it.

[paultvx]

Originally posted by WyrreJ
Gross yeah, but to net $2K per vehicle they

Was just an example.


people will pay stupid money for it.

And they do.

For example, I drove my bud's 82 yo grandmother's car today (97 Lincoln Mk8 with only 50k miles, 4.6L V8)... I've also done some maintenance work on it recently. That car is a total POS inside and out. Cheap plastic, cheap leather, you name it. I think her late husband paid 30 grand for this thing back in 98... it was a short-term lease return. 4 months ago it ran like a carb v-8 truck limping on 2 cylinders. It took us 3 hours and a lot of swearing to check and change the plugs. The work was a royal pain in the *****. It has the dumbest engine bay layout I have ever seen. After the new plugs were in, it still ran like a POS. Took it to the dealer and the dealer was amazed that we had changed all 8 plugs in 3 hours. It's a job they normally charge 4 hours for. He also mentioned that on the Navigator, it's even worse. It's a 5 to 6 hour job. 5 to 6 hours to change plugs?! And yet people are dumb enough to pay stupid money for them. So, after two days the dealer finally diagnosed the problem... a faulty plug wire. Guess how much a set of new wires came to... a whopping $250. Hell, a set of quality German made spark plug wires for the Audi A4 costs $80. The V6 VW I had the plug wires only costed me $75. The final repair bill came on the Lincoln came to over $350... and of course they try to convince us that the EGR needed to be replaced or it won't pass smog. Well, the car was tested at DEQ last month and passed with flying colors with the same damn EGR.

[Dallas4u]

I bet GM and Ford dealers make MORE money on service and parts than new cars, no doubt. Most money comes from used cars and the *****-ramming you get between your trade the the purchase. I LOVE looking at cars... but the negotiations and financing parts is a beeotch...

I read a little article in Consumer Reports (I know... many of you hate them for being under the corporate thumb), but they had a little investigative report on car buying in their 2002 Car Buying issue. They had people negotiate prices in person, over the phone, through internet-direct pages, and over the net using dealers "net special" deals. They found the lowest prices were actually had through the internet-direct websites like CarsDirect.com. I also know a number of people who have purchased new and used cars online and they have had great experiences (great as in not having to spend 4-5 hours at a dealer, having him bend you over, just to walk away knowing he will call you in a couple of days just to pull you back in and bend you over a second or third time... ba$tards).

Anyway, point is... I am sick of working and don't want to do the number of things on my plate for the day, so I think I may spend most of the day on this site while they record my internet usage through our proxy server.

[WyrreJ]

I read a little article in Consumer Reports (I know... many of you hate them for being under the corporate thumb),

What are you talking about? If there is any mainstream publication in this country that is free of the corporate thumb, it is CR.

[Tone]

I’ve got an AutoTrans Recalibrator for sale as Grant decided it is not that big an issue. $175 delivered in the continental US.

[Daver]

Car & Driver was the magazine that posted statistics they gathered on speedometer error from the factory:

http://www.caranddriver.com/article.asp?section_id=4&article_id=1906&page_number=1

-Daver

[Clyde]

Has anyone done any more research on the speedometer recalibrator since the last discussion? If so, how does it work? Good? Bad? I like the 18" wheels that came on my VX, but don't like the tires that fit them. I use my VX as a daily driver and the Duellers don't provide a good wear with my 100 mile roundtrip commute. I would like to use something with a better treadwear, but don't like to have my speedometer off calibration. I just can't bring myself to spend $100/tire for something that isn't a good quality tire in my opinion. So, I have also wondered about speedometer recalibration so I could use a different size of tire with a better UTQG (treadwear) rating.

[Maverick]

10 mph" tolarance " is pretty generous, many depts. only allow about 5mph from my experience, especially on local roads..

I checked my speedo with my old magellan (cheapo) gps, found it to be right on up to about 45 mph, and about 1mph off(fast) at 65 mph.. not bad.. it seems to get progressively less accurate the fast i go, but no measureable difference between 65 and 80mph..im guessing its probably about 1.5 mph off at that point, but cant tell without much more accurate equipment.. checking the speedo (and the other gauges if possible) is one of the first things i do when i get a "new to me" vehicle.. ive had a few cars that had gauges that were way off..

ive no plans for any radical tire size changes, but if i did, id definatly want the recalibration. running mental recalibration equasions while driving makes it more stressfull and takes concentration away from things like avoiding idiots on the road..

[Tone]

Grant has been running the Recalibrator he got from me for several months and hasn’t had any problems to my knowledge. I’ll get pricing for anyone interested but less than $150 if I recall properly.

[Clyde]

Hey Tone,

Does the recalibrator make it so you can adjust the speedometer to any tire size? Exactly how does it work?

I'm not looking to fit a radical tire size on my VX and I'm definitely not going to lift the vehicle or cut into body cladding to fit new tires. It's just that I want to have a properly adjusted speedometer with a different set of tires for the 18's (summer set of wheels). There are so many different tires out there that are better, in my opinion, that I could use if it weren't for throwing off the speedometer.

[Tone]

Yes, set with dip switches -email me if you want Grant’s most excellent install instructions.

[psychos2]

i have larger tires and the speedo is now off by 5 mph.checked with gps.i think my tire size is 31.5 inches. 2 inches bigger than stock. shawn

[Clyde]

Thanks for sending me directions on how the recalibrator works Tone. That gives me a lot better idea as to what and how it works. How much would one of these units cost out the door? Any information you could give me on price would be appreciated.

[t2p]

Clyde:
.
$100 for an 18" tire is relatively cheap ..... almost 'budget'.
.
You may want to consider the 255-55-18 size - this size will provide a little bit more rubber on the road (compared to the OEM 245-60-18), but the diameter is very close to the OEM 245-60-18 size, so there will be no speedo issues.
.
There are a number of quality tires available in 255-55-18 size -
you need to consider the type you are looking for: high performance ('V', 'Z') rated), or a tire with good tread wear characteristics. Not that both are mutally exclusive, but (generally speaking) a HP tire will provide better handling and cornering characteristics, but at the expense of wear. Many HP tires are also poor in snow. A tire with good wear will (again - generally speaking) will not provide the feel and handling of the high quality HP tires.
.
Pirelli, Michelin, Yokohama, Kumho (these come to mind quickly) offer HP tires in the 255-55 size. Most of these tires will probably run (approx) $150-225 per tire - with the Kumho being the low cost exception at around $100 per tire. The Kumho also has a higher tread wear number compared to most other (all ?) tires in this category - around 320 vs 220. I know the treadwear numbers can be misleading - but it is noteworthy.
.
I have 255-55-18 Mastercraft Courser HTR Plus (mfg by Cooper) tires on my VX. They are not what I would consider a performance type tire - they are an 'S' rated all season 'touring' type tire. They do have a decent tread pattern, raised sidewall to protect the rim, and are relatively quiet. They have a 460 treadwear number. They cost (approx) $100 per tire. If I faced a 100 mile roundtrip commute, I would consider these.
.
t2p