Speedometer?

[Planohummer]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by StarsnBars:
Not real sure there f5. I had the smaller 31's when I bought it and got the 33's as a upgrade. They hooked up there ------ told me my speedo was now caled for the 33's. I suppose it was just a plug in off there equipment. I wasnt standing there watching them. My dealer told me I should get the speedometer reclabrated so I did. I hope I didnt get shanked 50$ for nothing. </div></BLOCKQUOTE>
Dealers can recal speedos for different tire sizes available on the vehicle from the factory. Due to liability reasons, GM doesn't supply the info on alternate tires/wheels.
The big change on speedo readings comes from rolling diameter, not actual diameter. Try to remember your high school geometry(I can't) and look at the relationship between radius and circumference(rolling diameter). It's a geometric progression, which means that the net effect on circumference is a compound effect from radius change.
getting away from the $5 words-bigger tires change rolling diameter more than the diameter change. Shift points AND ABS braking are affected by tire size. The pulsating rate on ABS application changes according to the rolling wheel speed that the brake module reads.
Don't forget that the difference on speedo will be a % change, not a flat amount. 10% difference at 80 mph is twice what it is at 40 mph.

If you want, disregard all the above and go by any competent 4wd shop-Hypertech programmers any many others will let you input any tire size.