veloAR

According to the Michelin specs, the 255/45 more closely matches the stock 225/55/17 in terms of overall diameter and revs/mi. The more generic Miata calculator puts the 245/45 closer to stock.

I like the idea of an extra .2in of sidewall height the 255 gives, as I'm trying to strike a performance/comfort balance (hence the all-season route).

How much clearance do you have with the 8.5" rims, 255mm tires and et20 offset?

I'd be attempting to use 18x8, et35, with 10mm spacers. If my calculations are correct, this would give me a net et25 (5mm closer inboard), but with .5" narrower rim (12.7mm), the overall net would actually place the inner lip of my rim approx. 1mm farther outboard than the 8.5 et 20.

Does this sound right?

Oh, and the 255's are about $20 cheaper per tire at tirerack.

itacud

And, your calculations for inner lip distance are also correct. =)

markcincinnati

The 225 x 55 x 17" standard size allroad tire is said to be able to be upgraded to 245 x 50 x 17" on the factory standard wheel and not cause any problems (speedo, computer, etc.).

The wheel/tire size on the 4.2 allroad is "standard" @ 245 x 45 x 18" and it is possible to buy from Audi an upgrade (plus 1) wheel-- for the 2.7T version of the allroad -- which is 18" x 8.5" -- it has always been discussed (on this forum) that the plus 1 sizing for our cars is 245 x 45 x 18" -- are you saying that a more "correct" size would be 255 x 45 x 18"?

itacud

While 245/45/18 is the standard plus-1 sizing for our stock size, not all manufactureres build their tires to exact guidlines. i.e. One factory's 245/45/18 may be bigger or smaller than the calculated measurements, for a 245/45/18 tire.

Take Bridgestone's S03, for example. Their 245/45/18 = 26.7", and their 255/45/18 = 27" (diameter). The stock diameter is 26.8". So, you have a choice of being .1" shorter than stock, or .2" taller than stock.

Of course, it is even more accurate to compare revolutions/mile, rather than speced diameters. The % error is less, with revs/mile. Stock Revs/Mile are 776.

The only thing not to be used in choosing sizes are calculated diameters. You'd have to get all manufacturers to adhere to exact standards first! =)

skidmark

<ul><li><a href="http://www.tirerack.com/tires/Spec.jsp?make=Michelin&amp;model=Pilot+Sport+A%2FS ">http://www.tirerack.com/tires/Spec.jsp?make=Michelin&amp;model=Pilot+Sport+A%2FS </a</li></ul>

Ex-PzKW Cdr

Bridgestones SO 3 which are in turn about .25 bigger than Pirelli A/R 6 that came with the car. Fit is perfect with 8.5 18 ET20 Audi rims. At lowest setting car sits slightly higher than stock. Tires have performed MUCH better than expected. You really can't be 100% sure with published info. Manufactures sizes vary, one's 225 55 17 can be bigger or smaller than anothers, and can vary among the single manufacturers line!

skidmark

sure, now the stuff about 4/32 shaved is gone but it was there this afternoon : )

medievalAR

it definitely said "shaved 4/32", but it's missing now...it's a conspiracy!

skidmark

thanks! i thought i was losing my mind : )

veloAR

Thanks for the input. Sorry, I can't keep up with the sig name changes : )