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Determining Speedometer Error Due To Tire Size Change

27K views 18 replies 17 participants last post by  57buick 
#1 ·
When you change tire sizes, you will experience some speedometer error. Indicated speed may not be your actual speed.

So, if you are curious about how big of a change you will experience, go here:

Speedometer Error Calculator

Then, plug in your stock tire size (or your current tire size if you have adjusted your PCM for this using a programmer) and the new tire size to see what the change will be.
 
#6 ·
A programmer (Predator/Trinity) can be used to change the tire size in the PCM.

As far as going from 20" to 17"- lots of factors to consider, but the main ones are overall tire diameter, rim/tire weight, and rim offset (aftermarket rims). You can go from a stock 20 to a 17, keeping the overall diameter the same, and you'll have no problems.
 
#17 ·
My federal 35's are off by about 2mph at 65. According to my GPS and 1 mph at 40 according to the radar sign on my street. That was without the trinity.
 
#18 ·
Alfaobd is Android and windows software that will allow you to adjust for tire size changes on (I think) 13 and up years. Can also by obtaining your 4 digit PIN from a dealer parts department adjust the tpms setpoint, as well as program keys (fobik) and many other things. On 18 and up, FCA put in a security module that is supposed to prevent access to the rf module, but there is a bypass cable that can be had to still make changes.
 
#19 ·
You need to use the alfaOBD software and enter your "actual" tire size measured, you can measure your tires and convert to MM then you do some calculation (i googled the formula) and enter that direct and it will be dead on. For example my 37 inch tires with 40psi in them actually measure about 35.5, I don't remember what I calculated for MM but once I entered that and did the ABS reset and all that after it was balls on
 
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