What is it that I need to go exactly to get my speedometer and my mileage to read correctly after upgrading my stock 16" wheels to 18" wheels? Where do I get this magical part or gear? And is it relatively easy to install?

Please help.

THANKS!
Matt

 

That's a hard one to answer. If you purchased the correct size of rear tire to go along with your new 18" wheels, there shouldn't be any difference.

225/50 - 16 = 24.858" diameter stock
225/40 - 18 = 25.086" diameter will raise car .114"
235/40 - 18 = 24.401" diameter will lower car .228"
245/40 - 18 = 25.716" diameter will raise car .429"
245/35 - 18 = 24.751" diameter will lower car by .053, best for matching stock diameter.

255/40 - 17 = 25.031" diameter raises car .086" from above stock.
275/35 - 18 = 25.578" " raises car .360" "

 

I purchased 245/40 from TireRack.com - it was the size they suggested would be best so I didn't think I would have this problem. It looks like my car was raised .429" (the largest number that you listed). Bummer.

The circumfrence if the tire changed by 2.7" or 3.45%. Can I use this percentage to calculate my speed difference and mileage difference? It doesn't really matter. Just curious.

So now we know my new tire size, what do I do?

THANKS
Matt

 

My rears had 245/45-16 - I believe that is where the speedometer and milage readings are calculated from.

If you check out the following link it looks like the speed difference is fairly minor for speeds under 100mph: http://www.paspeedo.com/calculator.htm

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
245/45-16 4.3in 12.3in 24.7in 77.5in 817 0.0%
245/40-18 3.9in 12.9in 25.7in 80.8in 784 4.2%

That 4.2% also means that for every 12,000 miles I drive, I really drove 12,500 miles.

 

I think if you're within 5% you're o.k...... imho