Rev-Limited vs. Drag-Limited
A rev-limited car reaches its top speed at its maximum engine speed (i.e. redline) in top gear. In a rev-limited car, the power generated by the engine as it moves through its rpm range is sufficient to overcome aerodynamic drag. (A rev-limited car should not be confused with a car equipped with a rev-limiter. A rev-limiter is a device found on most modern cars which cuts off fuel above a certain engine speed.)
A drag-limited car reaches its top speed prior to its maximum engine speed in top gear. In a drag-limited car, the engine reaches a point in its rpm range where it cannot produce enough power to overcome aerodynamic drag.
With the exception of the 2500, all E3s with manual transmissions, in stock form, are rev limited. As many Bavaria owners will attest, their cars will pull all the way to redline in fourth gear. If you converted your stock transmission to a five-speed with overdrive, or if you have significantly lowered your final drive ratio, then your car is likely drag-limited. It is difficult to determine the theoretical top speed of a drag-limited car. In addition to the variables factored by this calculator, you must take into account the torque curve of your engine and the drag coefficient of your vehicle.
Why You Can't Trust Those Numbers on Your Tires
There are at least three factors relating to tire size which affect the accuracy of this calculator. One, the width and sidewall ratio listed on your tires are approximations. The actual measurements vary depending on the manufacturer and type of tire. Your 195/70 tires could actually be 192.3/71.1 tires; your 225/60 tires might be 226/62 tires. Two, the diameter of your tires gradually diminishes as your tires wear. Some tires can lose as much as 0.8 inches of their original diameter and still have some tread left. At 6000 rpm in fourth gear, a difference of 0.8 inches would equal almost 5 MPH. Finally, the effective overall diameter of your tire will change with temperature. The heat generated by high speed driving will have a marginal (but measurable) effect on tire diameter.
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