OK, I’m assuming that everyone that got the stock 19’s on their 6MT Coupe has the following tire sizes; 225/40-R19 on the front and 245/40-R19 on the rear. I can’t believe that I didn’t notice this earlier but if you actually interpret those numbers they you’ll notice that our rear tires actually have a larger sidewall that the fronts. (How? The first number is the width of the tire in millimeters, the second is the percentage of the first that dictates the size of the sidewall, and the third number is the diameter of your rim.) Here’s the raw data for each tire; which I got from http://www.miata.net/garage/tirecalc.html

Fronts (225/40-R19)
Sidewall: 3.5”
Radius: 13”
Diameter: 26.1”
Circumference: 82”
Revs/Mi: 773

Rears (245/40-R19)
Sidewall: 3.9”
Radius: 13.4”
Diameter: 26.7”
Circumference: 83.9”
Revs/Mi: 755

So, as you can see, because both of these use 40% of the width for the sidewall height, the rear tires are actually LARGER. Also what this means is that for each “mile” you drive (I’m guessing the speedometer and odometer are hooked to the rear wheels) on your rear wheels, your front wheels have actually traveled an extra 123 feet. Not a big deal you say? I’ve got 21k miles on my car, do the math and my front tires have actually traveled 489 miles MORE than my rears, all because of the tire size. So I played with some numbers and found that if I get 245/35-R19’s on the rear they actually end up being smaller than the fronts, but not by as much. However they are more closely matched as far as size goes.

245/35-R19
Sidewall: 3.4”
Radius: 12.9”
Diameter: 25.8”
Circumference: 80.9”
Revs/Mi: 783

My question is, has anyone else noticed this and bought different sized rear tires in an effort to get everything spinning at about the same speed, or am I just making a big deal about nothing. One nice thing about the 245/35-R19’s is that they are SO MUCH cheaper than the 40%’s

 

I couldn't of said this better myself.
Don't worry about it. Your tires are fine.