View Full Version : 18" wheels and odometer accuracy

02-12-2009, 12:57 AM
To those of you who have changed from the 17's to the 18's, and havent bought a hand held tuner to adjust the change, what are you seeing as to the change in speedometer and odometer accuracy.

Andy M
02-12-2009, 07:14 PM
I am curious to see what replies you get on this as I too am contemplating putting 18's on my Cobra.

For what it's worth though, as long as the rolling circumference remains the same (ie: you go with a shorter tire when you upsize the rim), there should be no change in odometer/speedometer. It is when you start changing the height that things begin to change because you are changing the overall circumference.

02-13-2009, 01:56 PM
Here is a tire calculator put in your stock size then the 18" size tire and it gives you the speedo difference. http://www.1010tires.com/tiresizecalculator.asp

02-13-2009, 06:58 PM
I run 18's and have no problems. when I calculated the difference,it was less than 1mph


John Himsel
02-14-2009, 09:15 AM
The wheel size 18s has nothing to do with it as stated above.....it is all about the diameter/circumference of the tire.

A 275/40/17 should have the same circumference as a 275/35/18.......the change will occur when you change profiles.

A 275/40/18 will be taller......and will make the effect on your gearing change from say a 3.55 to 3.50.

I had 335/35/17s = 26" tall with 4:10 gears. I wanted a little less gear than that but more than 3.73....and the 3.90 option from Richmond is noisy and was not an option. So went with 325/45/17 = 28" tall which made my gearing more like a 3.92.

I noticed about 5 mph with 2" taller tire