You are right, we need the complete drivetrain (engine, gearbox, differential, wheel diameter) to calculate from MRev's to mileage of the whole car. I agree with you, that MRev's is the right dimension for designing an engine. But most people want a car, not an engine. Every thing is referred to mileage (warranty, inspection intervals, lifetime etc.).
I hope you agree, that finally it would be the best, when we see the calculated mileage of the car (after fitting it together). A car is finally that, what is buildt in automation. Not only the engine (even if it is the most interesting task, the engine is the soul of a car).
Of course everyone drives a car in a different way, but even then you have to stick to speed limits, which are very similar all over the world. With these speed limits we can set up a "gear usage profile" (which gear is used how many percent of time). Out of this we can calculate an average gear ratio, which is finally used (with the wheel diameter) to calculate the predicted mileage.
For example, I would use a five speed gearbox with this weighting:
1. 5% (just for getting the car moving)
2. 5% (mainly for acceleration)
3. 20% (cruising in the city)
4. 20% (cruising in the city/country side)
5. 50% (cruising on the highway)
Of course that is just a suggestion, the weighting could be changed for the desired target country (USA is slower than Germany). That's a task of balancing.
Maybe the gamers tend to use a "long" gearbox, because this gives a high lifetime, but next they will recognize, that the long box doesn't fit to the engine characteristics. For example bad acceleration, or the maximum speed can't be reached. Everything not so easy

, but that's what makes Automation interesting.
That's just an overview of my idea of converting MRev's to mileage. If you want me to, I will write a complete document with examples.
Greetz,
Zeph