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So kind of a dumb question, but I'll ask anyhow. Is the stock computer programmd differently for the different optiona tire sizes? IE: V6PP with 255/40r19 (27.0 OD) vs base with 225/60r17 (27.6 OD)? I figure it must be, but that leads me to ask- is there any way to reprogam for different sizes without a tuner? Looking to run either 225/60r17 or 235/55r17 but I would like to have an accurate speedo (if possible). :bigthumbsup
 

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Yes, each tire size option would have a specific factory tune.

And I suppose the dealer could load the tune associated with that option onto another PCM but then it wouldn't match your original tire size when you switch back.

Why not just get 17" tires that are close to the original 19's and go with that?
 

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First, I have a GPS speedometer and the dash and the GPS usually disagree by enough that I disregard my dash and use my GPS speedometer when setting cruise control. Not to mention that the dash speedometer has very course resolution anyway.

This leads me to believe that the speedometer is not really calibrated to be correct for my tire setup. Most likely the speedometer calibration is for something like a 27.3 inch diameter wheel.

Second, let’s do the math:

80 MPH*27.6/27=82 MPH.

I think you will live if your speedometer reads 2 MPH higher than it does today.
 
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