Tire/Wheel Combination Calculator by: Bruce Leggett
Defaults are set for the BMW E9 family of coupes.
Change any parameter to explore alternative tire & wheel sizes:
Current Tire/Wheel: / -   Wheel Width in. and Offset mm.

Explore alternatives up to:

 / - Keep speedo error within mph
Note: no checking is done to determine if tire actually fits on the specified wheel or the tire/wheel within the wheelwell
Default diff ratios:3.45:13.25:13.64:1
 2800CS/CSA, euro 3.0CS/CSA, 3.0CSL3.0 CSi, 3.0 CSiLUS 3.0CS/CSA
Tire Size Ride Height Change (inch) True MPH when speedo reads For Diff Ratio & Trans Ratio Change with new wheel width of in. & offset mm.
mph % Error Top Speed @ rpm Effective Diff Ratio Inboard (in.) Outboard (in.) Track Width (in.)
/* Results dynamically written here */

Note that these are ideal calculations and don't account for things like drivetrain losses on top speed or the effect of shoehorning a wide tire on a narrow rim (or vice versa - yikes!).
Some collected wisdom:

Offsets:

You generally want to stay at the same offset as your stock wheels. Changing offsets can result in a change in scrub radius and torque steer (FWD and AWD only).

1. When changing diameters and offsets, your number one concern is to make sure the wheel will fit with your current suspension and bodywork. Every car is different, so check with other people who have your same car to determine what will fit with your brakes, suspension drop, body work, etc.

2. When going to a smaller offset wheel, toe-in must be increased (same as decreasing toe-out) to compensate.

3. Going to a smaller offset wheel will change the scrub radius. The only way to bring it back to stock is to increase the overall diameter of the wheel/tire assembly. This has nothing to do with wheel diameter, but everything to do with the rolling diameter of the tires.


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