In aviation, we use gallons per hour and that is a very useful metric. It allows us to plan our trips without having to do a lot of long division since multiplication is easier and less likly to have errors.

What you've proposed is the inverse of MPG scaled to unit values. Thus 50 MPG becomes 0.02 GPM (Gallons per Mile) and scaled to 100 mile units, 2 GPC. The problem is "C" in GPC is not a natural identifier for "100 miles" and "H" for hundred is ambigious with "Gallons Per Hour." But this suggests looking at Gallons per Hour.

The EPA tests have an average speed of 20 miles per hour City and 48 miles per hour Highway. Invert the EPA MPG and multiply by the speed and you'll have gallons per hour. For a Prius I, it would become:

• 45 mpg Highway -> 1.06 GPH, Highway (variable, 48 miles per hour)
• 52 mpg City -> 0.38 GPH, City (stop and go, 20 miles per hour)

This makes calculating the relative cost fairly easy knowning how long each trip takes in hours. It also solves the problem of leading zeros until cars start getting better than 200 MPG. It won't bring us closer to the metric system but it shares the advantage of simplifying the cost to operate a vehicle.

Bob Wilson

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