Speedometer recalibration: is it possible?

[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, 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.

[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.

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?

[AnalogVX]

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

[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? 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?