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Originally Posted by 3Hummer
...32" to 33" on his rig and it shows 78 when hes going 75 etc....
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I think it is the other way around; with larger tires speedometer will show less then the actual speed
you may want check one of many calculators, here for instance:
http://www.discounttire.com/dtcs/infoTireMath.dos
as Sewie said above, to adjust the speedometer, transmission shifting points etc you have to reprogram PCM. When I went from 32 to 33 I didn't adjust anything and all is OK. One of the reasons why I didn't step up to 35's was that I didn't want to bother with PCM adjustments which may have been needed after 32-35 step; the 32-33 seems marginal