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Old 02-11-2012, 01:59 PM   #17
Scooby Newbie
Member#: 164242
Join Date: Nov 2007
Chapter/Region: E. Canada
Location: Toronto
1992 SVX
2008 Spec B


This thread is hurting my brain. Mostly the OP though, thinking that 215/45 and 195/45 are the same rolling diameter.

However, someone explain to me why two different sized tires can't have matched diameters. Or, better yet, try to convince me that every Subaru running around has front/rear tires that actually DO have matching diameters. I know they're the same size, but think about it: If you inflate all your tires to the same pressure (as most owners will do, regardless of the door placard), then the heavier front end will compress the front tires more, resulting in a smaller rolling diameter. You could also look at how many cars are overdue for a tire rotation. Since our cars chew their front tires down faster than the rear you could easily have 3 or 4/32" difference in tread depth (which is a radial reading, so double it for diameter - 8/32 = 0.25"). If our cars were THAT sensitive to variances in wheel rotations our transmissions would be falling out every couple weeks.

I give you an example: The Nitto NT05 in 275/35 and 245/40/18. The 245 has an OD of 25.59", and the 275 is 25.63". Are you going to tell me that 40 thousandths of an inch difference in OD is going to hurt something? Really?

I'll also say that using tire size calculators are a good starting point, but one should consult the manufacturer's spec sheet for the final word.

Originally Posted by cwarfield0824 View Post
I'm doing a staggered setup of ESM 004's 17x8 in the front and 17x9 in the rear. I know in order to do this correctly you have to have the same rolling diameter for each wheel so you don't screw up the differential. I'm thinking about doing 215/45's on the rear, and was wondering is anyone knew what size I could run on the fronts. My friend mentioned I might be able to do 195/45 in the fronts. But I'm looking for second opinions. Thanks in advance!
Your friend is an idiot, and you need to learn how tire sizes are measured. Read the links the friendly moderator provided and educate yourself.
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