I replaced the stock tires/rims with 17" wheels and taller than stock all weather (Blizzaks). While I was doing the switch, I laid a blizzak+wheel over a stock+wheel, the blizzak combo is about 1/4" bigger in all directions. In other words, when you stood them up next to each other on a level surface, the top of the blizzak combo is about 5/8" taller at the top.

Is my speedo reading faster, slower, or so close to the same that it doesn't matter? And what about the odometer?

Question # 2, since this combination is much smoother and quieter, and the tread on the original tires is getting thin, I'm thinking of leaving these in place all summer in '06. Bad idea? Pros and Cons?

 

Bigger wheels = more distance traveled per rotation, so your spedo will be slightly under the real speed. Same with the odometer. You could work it out, you know the diameter of each wheel, so you can work out the circumference, and then find out the percentage difference.

If the distance traveled / circumference was 10% more then the spedo would be 10% slower, well, that's what I think. Am I right?

 

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

Enter your old tire size, new tire size and it will tell you the circumference differences as well as the impact on your speedo

 

Nice find

 

There is a wealth of great info here === http://forum.miata.net/vb/forumdisplay.php?f=12

 

Great help! Wow. Thanks a lot. I'm off by about 1.2% - speedo reading too slow, but that's only about 1 MPH at 70, so I doubt it'll land me in jail.

 

But it will be off 1.68 mph at 140 and that will land you in jail in most States.

 

Very good find. Personally, I'm off by 2.7%, but I entered info for the average person running 20s and if I chose the right numbers, they're going about 15mph faster...