Speedometer adjustment
Thread Starter
Safety Car
Joined: Sep 2002
Posts: 4,971
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From: The reason time exists is so everything doesn't happen at once
Speedometer adjustment
I'm considering a set of 18" wheels for the rear of my 93 coupe. The current tire size is 275/40/17 and the new tires would be 295/35/18. I calculate the difference in circumference as about 1 1/2". How much would this through off my speedometer and odometer? Is it worth adjusting it and how do I do it? On older cars you would change the speedo gear but I'm thinking on these cars some sort of reprograming would do the trick.
Race Director
Joined: Oct 2005
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From: Rolla Missouri
2021 C4 of the Year - Unmodified Finalist
St. Jude Donor '10 thru '25
You didn't say what tire your looking at. There are small differences between brands for the same size tire. What you want to do is find a tire that is as close as your original diameter as possible.
Spec's for Goodyear GS-D3
275/40 17 = 25.7 inches (811 Revs per mile)
285/35 18 = 25.9 inches (807 Revs per mile)
295/35 18 = 26.1 inches (801 Revs per mile)
It looks like the 285/35 18 is the closest fit to stock diameter, and will keep your speedometer pretty close.
Spec's for Goodyear GS-D3
275/40 17 = 25.7 inches (811 Revs per mile)
285/35 18 = 25.9 inches (807 Revs per mile)
295/35 18 = 26.1 inches (801 Revs per mile)
It looks like the 285/35 18 is the closest fit to stock diameter, and will keep your speedometer pretty close.
