Ok, stock tire size on my SE is 185/70/14, according to Tire Rack. If I bought 2 tires for the rear, in size 205/55/14, how much would that effect my gear ratio?

 

This is a tough question, because the height of the tires, although supposedly determined by the width divided by the aspect ratio, is not the same brand by brand.

 

I will do some simple math, and probably make myself look like a moron.......

185 / 70 = 2.64285

ok......now......

205 / 55 = 3.72727

so, this will definately have an effect on my final gear ratio. i know the car has 4.10's. but, tire height also plays a part in all this.

 

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

 

oh sweet, that link is very cool. But, I still am lookin for someone to come up with a gear ratio for me..........something that would be higher than 4.10 cause of the shorter tire? or would it be lower than 4.10?

 

bump.....

 

UniqueTII is correct, the maths isn't reliable because tire manufacturers obviously don't know how to use a ruler! But we can use it as a guide.
A 185/70x14 tire should be 615mm in diameter, a 205/55x14 should be 581mm, which is ~5.5% smaller. This would be the same as changing to a 4.34:1 diff. You should always aim for the original overall diameter; the closest profile for that width is 205/65x14 (Ø622mm).