Compare highway and city fuel economy.
Combining city and highway economy is the classic place people reach for a simple average and get the wrong answer. Fuel economy is a ratio with distance on top, so shares of distance have to be combined harmonically: the reciprocal of the sum of each share divided by its mpg. With 60% of miles at 36 mpg and 40% at 25, the correct combined figure is 30.9 mpg, whereas a straight weighted average of the mpg numbers gives 31.6 — small here, but the error grows as the gap widens. The gap itself is worth noting: city driving costs more because of idling, repeated acceleration from rest and the fact that regenerative braking does not exist in a conventional car.
Highway vs City MPG
Combined mpg = 1 / (highway share / highway mpg + city share / city mpg); annual gallons = miles / combined mpg
Because mpg is a rate with distance in the numerator. Averaging the rates over-weights the efficient one; you have to average the gallons per mile and invert, which is the harmonic mean.
Idling burns fuel at zero miles per gallon, and every acceleration from rest throws away energy at the next brake application. Hybrids close most of this gap, which is why their city figure can exceed their highway one.