Just curious, my coupe currently has the stock 18" wheels. If I upgraded to an aftermarkey 19", how do I make the adjustment for my odometer and speedometer. I know the difference won't be huge, but I'd like for it to be accurate nonetheless.

Also, for spare tire size. Can I just take my spare 18" wheel and mount it with a tire with a bigger side wall to make it's diameter equivelant to the 19" wheels I run? Anyone have any advise in this area?

 

Tire diamater = rim diameter + 2 sidewalls
sidewall height = width x aspect ratio

eg. 225/50/17

sidewall height = 225mm x 50% = 112.5 mm -> divide by 25.4 to from mm to inches.

total diameter = 17 + 2 x (112.5/25.4)

don't have a calculator handy, ez to put this into excel and play around.

 

http://www.miata.net/garage/tirecalc.html

 

Here ya go. I got some of this info from another forum member but have since added to it. Use it as reference. As long as you compensate for the plus sizing, the speedometer / odometer won't really be affected.

And no, I wouldn't suggest making any changes to the spare. As long as you have the same Overall Diameter, it won't make a difference regarding the spare.

BTW, I'm running 19x9.5s in front with 255/35/19 & 19x10.5s in rear with 275/35/19.

 

Now THAT is a nice little spreadsheet. Thanks for the information with that! I didn't even realize that the overall diameter probably wasn't even changing, just the amount of sidewall was being reduced.

Anyone have problems with bent rims from regular bumps with 19" wheels? Granted you are going to bend your rim if you hit a curb hard enough

 

Pot holes on our freeways is my biggest fear..........
going 75mph and hit a pot hole 3 to 4" deep your
done! Tire sizes, I usually go with the lower ratio
like from a 245/40 18 to a 275/35 19 seem to work
ok. Plus I generally order my tires from the TireRack
and I ask them if that is acceptable.