tire calibration question

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Mar 6, 2010 | 06:41 PM

97XLT4X4

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From: Round Lake Beach, IL

tire calibration question

so we recently got a F150, it came with 285/75/16's on it. we went back to stock (235/70/16). my question is this.... with out calibration, the speed would be off by 8 mph 68.2 versus 60 (285 VS 235). is there a way to know if it was re calibrated for the new tires? will a trip to the dealer help?

thanks
Juan

Mar 6, 2010 | 06:51 PM

shotgunz

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From: NC

Do you have a portable GPS?

By my calculations, if you're showing 60 with the 285s, you'll show 52.9 with the 235s.

It's a simple ratio (2620 is the unloaded circumference in mm of 285s).

60/2620 = x/2310

The dealer should be able to recalibrate for the new tire size.

Mar 6, 2010 | 07:16 PM

shorebird

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From: Long Beach, Ms.

Get on the interstate highway and check your odometer as you pass the mile marker. Give it 5 or 10 miles and see how far off your odo is off.
I'll bet that the previous owner did not change the calibration when he changed to the larger tires.
You are reducing your tire size by 11.81% from 285/75/16.
Use the calculator here; http://www.carbibles.com/tyre_bible_pg4.html
The 285/75/16 tires are 13.39% greater than the stock size you are putting on.
Therefore the mileage shown is actually 113.4 miles for every 100 shown if the previous owner changed the tire size when the truck was new and he did not recalibrate for the new size.

Mar 7, 2010 | 06:00 AM

Joe Finn

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From: DC

I think that '97 (and maybe '98) used a mechanical speedo. I'd bet the previous owner didn't go to the trouble to change speedo gears. The newer trucks are calibrated via the PCM.

Mar 7, 2010 | 07:39 AM

97XLT4X4

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From: Round Lake Beach, IL

thanks for the information, I've always used the link below for tire size change, I'll use our gps and see what it says, seems like its right but sometimes you can never tell.

Tire size calculator

-Juan

Mar 7, 2010 | 10:50 AM

galaxie641

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From: SE Wyoming

As said above '97 and '98s require the VSS gear to be replaced so don't buy a tuner or try and have the dealer recalibrate the ECU as it won't work. I went from 235's to 285's and it was almost 15% off easily noticeable with the odometer if nothing else.

Mar 7, 2010 | 11:22 AM

shotgunz

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From: NC

Quote:

Originally Posted by 97XLT4X4

I used the same calculator and came up with the same number as I did (52.9). I put the 285s as stock and the 235s as the new tire.

In your first post, I think you reversed stock and the new tire.