match tire circumferene front to back?

[coloradoporsche]

Hello,

I am wondering how important it is to match the tire circumference from front to back wheels. My stock tires (205/55 and 225/50) are almost exact. But I'm considering various combinations that might be off by, say, 9mm in sidewall, resulting in a different height and circumference.

Does it matter if the front matches the back?

Thanks!

[Jack Olsen]

It really doesn't matter very much at all, except maybe visiually. What's more important is the overall diameter of the tires you choose, especially on the rear. This will effect your car's overall gearing, as well as your speedometer/odometer accuracy.

A big disparity will change the rake (leaning forward or back) of the car.

[scylla6]

I generally like tires that are the same diameter front to rear. Try using this formula and you can figure out the diameter of any size tire.
DIAMETER = (width)(aspect ratio)(2) divded by 25.4 (+ wheel size)

for 205/55/15's it would be;

(205)(.55)(2) divided by 25.4 +15 = 23.878".

This can come in handy when mixing and trying to match up different tires. On an early 911 I like to try and get around 25".
Tom

[Jack Olsen]

Or if that much math makes for a bad humming sound in your skull, use an online tire size calculator.

[Noel]

BTW, you don't mention what kind of car you have. If it is a two wheel drive car, different diameter tires are not a huge deal. However, if it is a C4 (964 or 993), then you may need the same diameter front and rear as it may damage the all-wheel drive.

[coloradoporsche]

Thanks for the info, guys. I have a 79SC. I really appreciate the feedback.

Martin