***996 C4S/TT Tire Fitment Question ***
 

I know this topic has been discussed countless times and i tried a search but didn't get too far...

Currently on stock C4S/TT 18inch 'twists' and am getting an extremely good deal on a full set of new Bridgestone Potenza RE050As:
225/40/18- Fronts
295/35/18- Rears

Will this setup work or will i have diff/trans problems long term? I've read since the back is slightly 'taller' than the fronts i could have an issue...i know OE is 295/30 but will these be a big problem long term? Anyone running this setup? Tire gurus please let me know!

Thank You!

There is about 4% difference in circumference in those two tires. I would not run them in an AWD car!

ok thanks MBailey. Whats the equation to find the circumfrence given the tire numbers? ...sorry for such amateur questions

Here is a great calculator for this:

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

Not a bad question at all! You can use the above calculator or published revolution/mile specs on each tire. Iam pretty confident the proposed configuation would pop your differential...

Danger danger. ... Never, never use any online generic 'tire size calculator'
if you are dealing with close tolerances, such as trying to prevent rubbing
or if you need identical front and rear diameters like for the Porsche AWD
cars. They all mean well, and are simply designed according to what the
tire size nomenclature is *supposed* to mean, but in the real world there
is a wide range of variance between what a tire is designated as and what
it's real dimensions are. For the real facts you have to go to the tire
manufacturers spec sheet. I have seen two different 245/45-16 tires differ
in diameter by 1/2 inch *from the same manufacturer*! They were just
different model tires. Recently one tire manufacturer stopped making it's
305/30-18. Now they list a 315/30-18. *It's the same tire*! They just
decided that the actual tire dimensions were closer to the 315 meaning,
so after 6 years they just changed what was printed on the sidewall.
To let you know how poor a ballpark answer they provide. Go to tirerack.com
and look at the spec sheets for 225-40-18 tires, and you will find diameters
varying between 24.8" and 25.5". The latter value is taller than the average
235-40-18!

225-40-18 tires:
Hoosier A6 - 24.8"
Bridgestone expedia S-01 - 24.9"
...
Michelin Pilot Sport - 25.3"
Dunlop Sp Sport 01 - 25.5"

Now look at the supposedly larger 235-40-18 tires, eg:
Dunlop Direzza Z1 - 25.1"

I'm saying that however helpful these generic formulas are trying
to be, they are quite easily capable of being way wrong, precisely
when it matters the most to be right.
Joe

Joe is correct, but the calculator I posted will tell you the basic size range that you should look for to get as close as possible. I don't really share the concern of a .1 or .2 difference in diameter because our cars are prone to larger differences through the wear cycle of the tire. The 1/2" - 3/4" range is concerning though. I am really surprised at those numbers

If you could get the same tire in 295/35/18 and 225/45/18 they would be within 1% of same circumference according to spec table. This would probably be safe.

Had not considered that. Back tire might rub too.