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Originally Posted by NightHawk756

Hmmmmm......interesting. That difference creates a 5-6 mph difference. Just seems like alot for what seems like a small difference in height.

For some reason I couldn't wrap my mind around the concept that going from a 17 to an 18 would be smaller. But thinking how tall the stock tires are on the new ones, it's starting to click now.

yeah its very important to figure out your overall diameter and maintain it as close as possible when stepping up wheel size. Hell its even important when you go to a wider tire.

It (1.5" shorter tire) actually should have created a larger difference in his speedometer than what he is showing. I am ass'uming that either the FRPP tune is either wrong or is limited in what tire size values you can input and they just a size that was close to what he has on the car.

Ray,
89 Hoss no longer has the car that he installed the FRPP tune in but I don't think he had any major speedo issues. He ran a 275/40/18 on his car