Miles per gallon? or Miles per Dollar? a better way to measure fuel efficiency?

We've talked here before on numerous occasions about how the average fuel efficiency of new cars and trucks hasn't really changed much since the late 1980s. We've also discussed the fact that simply mandating that car makers produce more efficient vehicles is not enough when fuel prices are low enough that drivers aren't yet willing to give up their bigger more powerful vehicles. Since American politicians clearly don't have the political backbone to drive up the cost fuel through tax policy, another approach is clearly required.

In a recent editorial piece, writer Lawrence Ulrich proposed a simple measure that might help drive home the cost of driving to American consumers. Since the efficiency of most cars doesn't change radically over time and many people pay for fuel at the pump with credit card rather than cash, they tend to be somewhat less aware of what's happening. However if instead of using the relatively static metric of how many miles you get from each gallon of fuel, you switched evaluating the miles you can go on a dollar's worth of fuel, a lot more volatility would be apparent. By dividing the MPG by the unit cost of fuel to get miles per dollar, you would get a live measurement of efficiency that more closely tracks cost. It's an intriguing idea, although it's not clear how practical it is. For it to be useful people would actually have to do the calculation each time they fill the tank. Or else in vehicle displays of mpg would need a way to input the dollar cost each time you got a fill-up (in-car computers that can calculate these numbers on the fly do exist, at least in test vehicles). Chime with you thoughts on this one and ideas of how to make it work.

[Source: MSN Autos, thanks to JJ for the tip]