SLPGT00

i orded 18in rims for my mustang will i need to get a programmer to input the larger tire? will my speedometer be off>>?? thanks for your help

matt

TimmyGt

depends on the tire size!

RAZER

It will be off just a tad not so much that you need a programmer. If you were putting in gear's then yes but you are OK.

JHonor

You can do the math, and as long as it's a size that doesn't throw off the wheel and tire's aspect ratio (height), it won't change the speedo. But it only works for certain sizes; like mine are 275/35-18, which is right around stock give a couple millimeters. But if you change the final drive gear, you might have to get a new speedo gear, or a calibration.

dirtboyz69

i have 18s on mine and its off by like 5 miles an hour over what your speedometer tells you

VARifleman

He doesn't have a speedo gear, the only way to adjust the spedometer for tire size or gears is through a tuner. Here is a calculator for tiresizes: http://www.miata.net/garage/tirecalc.html

If you run a 275/30ZR18, then you would be only 0.1% off.