: Recalibrating the speedo 05 STS V8

04-27-09, 03:28 PM
My car came with 17 inch wheels and I am looking to upgrade to some nice 18 inch sport caddy oem wheels. Can you please tell me if the dealer can recalibrate the speedo easily or about how much it will cost.


04-27-09, 03:49 PM
No need to. All you have to do is 'plus' size your tires. By this I mean if your stock tire is a 235/50X17 you could do a plus 1 in which you will be using 235/45X18 tire. By going down in ratio, the overall tire diameter will remain the same. (the numbers I used are arbitrary and other numerical combinations can work; see your wheel/tire dealer)

04-28-09, 04:57 PM
1010tires dot com has a nifty tire size calculator (sorry, cant post URLs yet). Put the original size in the Stock Tire Size form and what you want to compare in the Compare Tire Sizes form. Using C&C's suggestion, you'll find that the diameter (what your trying to keep similar to the original) is more than 3% less which makes your speedo read 3% too fast than you are actually moving. There are closer combinations: 235/50R18 is only 0.3% larger than original which makes your speedo read 0.3% too slow - and that's VERY close to original.

Note that I've never done business with 1010Tires and I'm NOT recommending that you do - I just love their caclulator!