I think there is a certain amount of logic in basing it on power to weight as it is that ratio which will ultimately determine how fast a car is.
For example many lightweight sportscars like Caterham, Lotus etc are relatively low powered, but due to their lightweight they are bloody fast.
The new Caterham 500 has 263bhp and weighs half a ton, the Bugatti Veyron has 1000bhp but weighs closer to 2 tonnes. Power to weight is almost exactly the same. Acceleration will be similar. The Bugatti will ultimately march on to a higher top speed but that is more to do with its better aerodynamics.
I don't think it can be based on power alone, in terms of finding a way to base it on the cars performance power to weight figures are probably the best method.
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