ryoung

<center><img src="http://pictureposter.audiworld.com/17157/a8_tires.jpg"></center><p>
A few months ago, I created this Excel file (now a .jpg) to help me select a new wheel/tire package for my S8. It lists tires by size that you might consider mounting on an A8/S8, but while keeping speedometer error, fender clearance problems, and cosmetic issues to a minimum. I think the table is reasonably self-explanatory, but a few comments

1. The horizontal dotted lines group results in 1% increments of speedometer error. I wanted to stay within +/- 1%, and to keep the tire's radius within about 1/8", of the original 245/45-18.

2. Depending on the manufacturer, a tire's actual dimensions do vary both + and - from these theoretical calculations.

3. The load rating of a tire is more-or-less proportional to the amount of air in it, hence % volume difference from the OE S8 tire. I didn't want a front tire that would be overloaded. My 255/35-20's at 32 psi are probably approaching the limit, but running 40 psi in the front tires more than compensates.

4. The approved and design rim width was based primarily on data from Bridgestone (if I recall correctly). Note that with the same width tire, the rim recommended gets wider as the tire profile gets lower.

5. All else being equal, a shorter sidewall height makes the tire more responsive; taller makes the wheel less vulnerable to pothole damage and improves the ride.

RCBENZ2

I don't understand the design rim statistics. In the stock s8 the design rim is 8". The Michelin Pilots on the line above has a design rim of 9.5". Does thim mean the Pilot's won't fit on the stock wheel, or that they shouldn't be fitted because the design rim is too narrow for the tire?

ryoung

... they shouldn't be fitted because the rim is outside the range of approved rims; or yes, the 8" rim is too narrow for that tire size.

In each case, the design rim is the recommended rim width for that specific tire size. Also, if you go to a tire manufacturer's web site, the tire dimensions they quote will be for the tire mounted on a rim with the design width.

You might be able to physically mount a 275/40-18 Michelin Pilot on an 8" rim, but the recommended width for this tire falls between 9.5" and 11". Too wide a tire on a narrow rim will distort the tread, preventing a flat uniform contact patch, and it may damage the tire. [A 245/45-18 Michelin Pilot is certainly OK on an 8" rim.]

PaulW

Randy......you are the guru.....thanks a million!

Wilfred

Seriously considering the S-03 Pole Positions any day now. Would love to go to 19" or 20" from 17".

ryoung

Actually, it won't be difficult to make a similar spreadsheet for the A6 4.2; I just need to "re-zero" with the OE A6 tire, add some additional sizes, and re-sort.

Are there any particular sizes/widths that you'd like me to be sure to include? Let me know ... I'll revise the spreadsheet and post it in the A6 forum.

Wilfred

I figure the 17" tires already get swayed by ruts and grooves in the road. Might as well get the widest contact patch I can. As I said, I'm most interested in the S-03 pole positions. I'm concerned about clearance with them, but would like to take it as far up as I can without problems. 20" would be nice, but I don't want to scrape in heavy shock motion or in full lock turns.