drewk86

I've been looking to purchase an aftermarket or second hand set of wheels and tires for my 88. My question is this: What's the correct way to re-calibrate the speedometer / odometer for the tire and rim size changes?

Thanks

don hall

http://tire-size-conversion.com/tire-size-calculator/

96GS#007

Depending on specifics you can change one or both of the speedo gears (one on the output shaft and one on the VSS) or you can tweak the tune.

Tweaking the tune is far easier and can take into account subtle changes in tire size which changing the gears normally does not.

8a8mfh

You need to change the speedo gears. Doing it through tuning on an '88 is limited.

Go to this link, it has complete information, be sure to scroll down to the 700R4 speedo gear section.

http://www.tciauto.com/tc/speedomete...lculator#700R4

5abivt

generally if you stick near the overall diameter of stock, moving up to 18/19 and even 20s you can stay near the overall diameter and not throw off the speedo at all or by a few mph at most.

drewk86

Thanks for the info everyone. So will the larger wheels/tires just affect the speedometer or will it also throw off the odometer?

WVZR-1

It can get very complicated depending upon what you have presently (axle ratio, tire size) and what you intend to purchase for replacements. It used to be not particularly expensive and parts were readily available. It's now very complicated to do it mechanically with the VSS, the drive and the driven combination.

Assuming your '88 hasn't had an axle ratio change or other modifications differing from stock it could be somewhat useful to confirm the speedometer/odometer is currently correct. Today that's very easy with GPS and measured miles marked on most highways. I'd confirm that it's correct and probably shop wheels/tires that might be very close to the dimensions of what you've got currently if it's correct.

If it's by chance maybe not correct shopping something that might correct it or get it closer with tire dimensions might be a good move.

Using a DRA which I never used to suggest it's quite easy and today maybe actually substantially less expensive than a mechanical conversion.

Using a GPS check the speed at 40, 50, 65, & 75 on the cluster and note the GPS reading for each. The reason for the "random" check is you're looking for a "percentage of error" NOT just a MPH it seems to be off. If it's not correct the percentage should remain the same at all of the checks.

What is your current tire size and axle ratio?

Both speedo & odo will have the same percentage error.

playsdixie

I think your rpo codes show what type of speedo gear you have , and then decide if you need to change it.....I went from stock 16 to 18 with lower profile tires and didn't notice much difference....

5abivt

Whichever way it affects the speedometer it will affect the odometer. So if it's a taller overall diameter you will be rotating a few % slower to achieve the same speed so the odometer may read a few % lower over time. hardly noticeable.

Again it's simple. Not rocket science. Go to tire rack .com and check the specifications on the tire. Look for overall diameter. When moving up in rim sizes generally the profile gets lower to maintain the overall diameter. If you're within an inch I wouldn't even think twice about it.

bjankuski

Is the overall tire diameter the same? If it is no change will be required. If you must change the speedometer then use the Dakota digital sg5 convertor and wire it in, by far the easiest way to correct the speedometer.

Churchkey

@ tirerack.com you want to compare tire revolutions per mile then figure the percentile difference between the two examples. The difference is the percent your speedometer will be off.

Revolutions per mile are figured by the tire manufacturer using static load radius also referred to as "squat radius". Static load radius is the working radius of the tire bearing the vehicle load. For the same size tire the revolutions per mile vary somewhat between tire companies depending on tire construction. For a streeter car = close enough.

I changed from 275-40-17 to 285-40-18 the speedo difference IMHO was negligible & did not warrant correction.
GPS indicated:
Speedo @ 60mph = 61mph actual
Speedo @ 80mph= 83mph actual