Chevy Impala Forums - View Single Post - Wheel sizes? 16 and 18 in some years, 17 inch also?
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post #13 of (permalink) Old 07-22-2019
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I went from 27.30" 235 50R18 Goodyear tires to 27.70" 245 50R18 Michelins. The larger tires have only slightly taller 0.20" sidewalls or 0.40" + on the diameter if you like.

Even slight changes like the above have very definite repercussions in the speedometer/odometer settings along with the ABS/Traction-Control system.

My new tires changed the speedometer that read consistently @ 3MPH higher than actual to @ 0MPH higher on two "your speed is" signs in the area.
I could leave the speedo as is and be aware I didn't have a 3mph buffer in the Impala. However. The revs per mile change from 740 to 730 that caused the speedometer to be more accurate also made the odometer read lower miles than actual.
I changed the tire diameter in the PCM and the ABS as soon as I got home from the tire shop a few miles away so my displayed mileage is largely unaffected.
Vehicles that are commonly changed to much larger tires, like My pickup trucks, are likely much higher mile rigs than the odometer states even tho nobody has purposely tampered with the odometer.

You don't need to go to the manufacturer to get the nominal tire diameter and other specs based on common metric tire sizes. This little web widget will allow you to compare two different tire sizes.

This is how you fix your rig if you've changed your tire sizes...

2009 Impala SS
"Speed never killed anyone. Suddenly becoming stationary... that's what gets you." Jeremy Clarkson
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