Miles per gallon is a trap.
Trade a 15 mpg truck for a 20 mpg one and you save about 167 gallons over 10,000 miles. Trade a 40 mpg car for a 50 mpg one — the same 5 mpg gain — and you save only 50 gallons, a third as much. Fuel used is proportional to 1/mpg, not mpg, so identical mpg gains save wildly different amounts of fuel depending on where you start.
Before mpg₁
1/mpg₁ ∝ ΔF
10 mpgstarting fuel economy60 mpg
After mpg₂
1/mpg₂ ∝ ΔF
10 mpgupgraded fuel economy60 mpg
Distance D
D ∝ ΔF
5,000 mimiles driven per year20,000 mi
Try
Fuel used over a fixed distance D is F = D/mpg — inversely proportional to fuel economy, not linear in it — so equal mpg gains are worth wildly unequal amounts of fuel depending on where they start. Psychologists Richard Larrick and Jack Soll named this the "MPG illusion": people misjudge mpg as if it scaled linearly, when it's the reciprocal metric — gallons per mile, or litres per 100 km as used in Europe and Canada — that scales linearly and can be honestly compared or averaged. The distance slider above is an annual-driving assumption, held fixed while comparing before/after mpg; real annual mileage varies by driver.