[PaleMelanesian]

The epa combined is weighted 45/55%. I don't remember which one is weighted more, but rounding it pretty much cancels that out anyway.

To calculate it, figure the gallons used in each and then divide by the total miles. Example from my car (33 city 38 highway)
25% city = 25 miles @ 33mpg = 25/33 = .76 gallons
75% highway = 75 miles @ 38 mpg = 75/38 = 1.97 gallons
Total = 100 miles / 2.73 gallons = 36.6 mpg.