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.

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. 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.)

[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.

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

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.