I was just wondering what people mean when they say that a certain sidewall size is the 'correct' size for a specific rim size. I have 17" rims and I have 45's in the back and 40's up front but I've heard people say that 45 is the correct size. So I was wondering if there are any adverse effects I should be worried about when it comes to having a thinner than 'correct' size tire on the rim. Inform me!

 

Your speedometer will be higher that you are actually going and you add up "undriven miles".
-Cyrus

 

Maybee you should rotate the 45 to the front so you get the correct milage and speedometer.

 

Bingo.
-Cyrus

 

undriven miles!?? thats whack!

 

The shorter 40 sidewall will ride a bit harsher than the taller 45. Those numbers are a percent of the width of the tire, giving you the height. So a 205/45 tire will be shorter than a 235/45 tire. The shorter sidewall also might give you a teeny bit sharper handling, but then again this might also translate into more abrupt break-away at cornering limits. Since they're shorter too, they offer just a few millimeters less cushioning to your rims if you smack a huge pothole.

Unless your front tires are one size wider than your rears, they will be slightly too short. This makes the engine spin faster down the road and the speedometer read faster than you are actually going (as mentioned). However, keeping width the same and going from 45 to 40 usually isn't a big deal to sweat about. Go here and plug in the proper size tire (or your rears) in the first box, and the front 40-sized tires in the second box to see how bad it is: www.miata.net/garage/tirecalc.html Probably won't be much, as is the difference in the tires' performance.

 

Hmm I checked that site out and according to that my speedo is 3.7% off. Well thats not so bad I guess. Just as long as nothing real bad is going to happen. I can take the extra road noise just until I get a new tires...