going to smaller rims and snow tires-help

[09SpecialEdition]

im going from my stock 17's to some snows on 16s or even 15's, how do i make sure i keep my speedo accurate as well as odo.

im sure theres another forum on here about this i just couldnt find it....

thanks!

[Ssej]

Do you have a GT or Spec B? If so, you won't be able to put smaller than 17" wheels on due to the size of the front brakes.

[slushboxgt]

Hi,

First of all, I don't know if you would have sufficient brake clearance with 15's, even a non GT? I'm sure someone else who knows will chime in.

The most important point when reducing rim size is to increase the aspect ratio, and you will probably also have to go one size smaller on the width. Aspect ratio is the sidewall height (the height from the rim to the tread) expressed as a percentage of the tread width. It is the 50 in 205/50/17, the 205 is the tread width, and the 17 of course the rim size.

You increase the aspect ratio if you go with a smaller rim size to maintain the same total outside tire diameter compared to what came on your car. If your stock tires are for instance 205/50/17, you would probably go for something like a 195/60/16 for your snows. This however is only a rule of thumb, you may have to tweak a bit by going up or down a size depending on the brand.

You can get the outside diameter of the tires that came on your car, as well as any snows that you may be looking at on the Tire Rack web site. Amazing how the outside diameter can vary so much from brand to brand on the same size tire.

 

Good luck,

Paul

[amp0412]

this website calculates the outer diameter of the tire based on the tire size. you can use it to figure out what size would be best.

 

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