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A8/S8 Tire Size Options
By Randy Young


The follow chart 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. The table is reasonably self-explanatory, but a few comments:

  1. The horizontal color groups 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 38 psi in the front tires more than compensates, and I'm running 34 psi in the rear.


  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.


  6. In the front, 255 is the largest width tire I've personally seen on an A8/S8. The MTM web site hints that a 265/35-20 will fit on an 8.5" rim, though 9.0" is the minimum recommended rim width for this tire. Anything wider than 265 probably requires fender modification.


  7. The rear has much more clearence for a wider wheel/tire; note that the "A8000" uses a 305/25-19 on an 11.5" rim! Unfortunately, installing wider tires in the rear will aggravate understeer, the last thing an A8/S8 needs.

Use this Miata Tire Size Calculator to determine any changes in diameter or width that is not documented below.

Tire Size Sidewall Ht., in. Dia. in. Radius Delta Width Delta Approved Rims Design Rim Volume Delta, % Speedo Error, % Notes/comments
305/25-19 3.0 25.0 -0.84 2.36 12.0-13.0 12.0 -15 -6 "A8000" rear, 11.5" ET41.5
245/35-19 3.4 25.8 -0.46 0.00 8.0-9.5 8.5 -22 -3 "A8000" front, 10.0" ET35
265/38-18 4.0 26.0 -0.37 0.79 9.5-10.5 10.0 -3 -3 Avon slick, 265/650-18
255/30-20 3.0 26.0 -0.33 0.39 8.5-10.0 9.0 -26 -2
255/35-19 3.5 26.0 -0.33 0.39 8.5-10.0 9.0 -15 -2
255/40-18 4.0 26.0 -0.32 0.39 8.5-10.0 9.0 -5 -2
255/45-17 4.5 26.0 -0.32 0.39 8.5-10.0 9.0 4 -2
265/30-20 3.1 26.3 -0.21 0.79 9.0-10.5 9.5 -19 -2 No tires available in this size
265/35-19 3.7 26.3 -0.19 0.79 9.0-10.5 9.5 -8 -1 No tires available in this size
265/40-18 4.2 26.3 -0.17 0.79 9.0-10.5 9.5 3 -1 Pzero C
275/30-20 3.2 26.5 -0.09 1.18 9.5-11.0 9.5 -13 -1
275/35-19 3.8 26.6 -0.05 1.18 9.5-11.0 9.5 0 0
225/60-16 5.3 26.6 -0.03 -0.79 6.0-8.0 7.0 7 0 Audi A8 OE, 7.5" rim
245/50-17 4.8 26.6 -0.02 0.00 7.0-8.5 7.5 9 0 Few tires available, but one is the Pzero
275/40-18 4.3 26.7 -0.01 1.18 9.5-11.0 9.5 12 0 BFG KD
245/45-18 4.3 26.7 0.00 0.00 8.0-9.5 8.0 0 0 Audi S8 OE, 8" ET48
245/40-19 3.9 26.7 0.02 0.00 8.0-9.5 8.5 -9 0
285/30-20 3.4 26.7 0.03 1.57 10.0-11.0 10.0 -6 0
225/55-17 4.9 26.7 0.03 -0.79 6.0-8.0 7.0 1 0 Audi A8 option, 8" ET48
245/35-20 3.4 26.8 0.04 0.00 8.0-9.5 8.5 -19 0
285/35-19 3.9 26.9 0.09 1.57 9.5-11.0 10.0 8 1
225/50-18 4.4 26.9 0.09 -0.79 6.5-8.0 7.0 -6 1 Winter, but no tires available in this size
285/40-18 4.5 27.0 0.15 1.57 9.0-10.5 9.5 21 1 CEC recommended (rear only?)
255/35-20 3.5 27.0 0.17 0.39 8.5-10.0 9.0 -11 1 8.5" ET45, or 9" ET39
255/40-19 4.0 27.0 0.18 0.39 8.5-10.0 9.0 -1 1 '04 A8 option on 8.5" rim
255/45-18 4.5 27.0 0.18 0.39 8.0-9.5 8.5 9 1 '02 A8, MTM on 8.5" rim; '04 A8 option on 8.5" rim
270/43-18 4.6 27.2 0.23 0.98 10.5-11.5 11.0 17 2 Avon slick, 270/690-18
235/55-17 5.1 27.2 0.25 -0.39 6.5-8.5 7.5 11 2 >> '04 A8 OE, 8" rim <<
235/50-18 4.6 27.3 0.29 -0.39 6.5-8.5 7.5 4 2 '04 A8 option, 7.5" & 8" rims
265/35-20 3.7 27.3 0.31 0.79 9.0-10.5 9.5 -4 2 MTM on 8.5" rim
265/40-19 4.2 27.3 0.33 0.79 9.0-10.5 9.5 8 2
265/45-18 4.7 27.4 0.35 0.79 8.5-10.0 9.0 19 3
245/40-20 3.9 27.7 0.52 0.00 8.0-9.5 8.5 -5 4




Randy Young's 255/35-20" tire and HRE wheel on his '01 S8. Notice the awesome brake upgrade!

Contact Randy via email!


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Technical & Maintenance  |  Upgrades & Style  |  Used A8 Buyer's Checklist

Marketplace  |  On Track  |  Gallery  |  Links |  Contact |  Home