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Fuel Efficiency Math: MPG, Cost per Mile, and Why Small Numbers Matter More

MPG feels like a simple number, but comparing fuel savings by MPG alone can genuinely mislead you β€” the math underneath behaves less intuitively than most people expect.

MPG = miles driven Γ· gallons used

The most accurate way to measure real-world MPG is the fill-to-fill method: fill the tank completely, record the odometer, drive normally, then divide the miles driven by the gallons it takes to fill up again.

A US gallon and an imperial gallon are not the same size

A US gallon is about 3.79 liters, while a UK/imperial gallon is about 4.55 liters, roughly 20% larger. The same car can show a noticeably higher "MPG" number in UK-style ratings purely because the gallon itself is bigger, not because the car is actually more efficient.

Cost per mile = fuel price per gallon Γ· MPG

To translate efficiency into an actual cost figure, divide the current price per gallon by your MPG. A car getting 30 MPG with $3.60/gallon gas costs about $0.12 per mile driven.

Official EPA/window-sticker ratings assume ideal conditions

Real-world driving, hard acceleration, cold weather, air conditioning, highway speed, underinflated tires, and city stop-and-go traffic all pull actual fuel economy below the EPA-estimated combined figure, sometimes by a meaningful margin.

Improving a low-MPG vehicle saves more fuel than improving an efficient one

Going from 10 MPG to 20 MPG saves far more gallons over the same distance than going from 40 MPG to 80 MPG, because MPG is not a linear measure of fuel used; gallons-per-mile (or per 100 miles) is the metric that actually scales the way fuel savings do.

Why MPG is a misleading metric for comparing savings

Because MPG is inversely related to actual fuel consumption, equal MPG improvements do not translate to equal fuel savings. A jump from 10 to 20 MPG cuts fuel used in half over a fixed distance, while a jump from 40 to 50 MPG only trims it by 20%, which is why fuel economy researchers often argue gallons-per-100-miles is the more honest comparison metric.

Why your own fill-up math beats a dashboard readout

A car's onboard fuel economy computer estimates MPG using internal sensor assumptions that can drift from reality over time, while the fill-to-fill method is a direct measurement based on the fuel and miles actually used, making it the more reliable number for tracking real efficiency changes.

Frequently Asked Questions

Why would two cars with the same MPG cost noticeably different amounts to drive?

Fuel price varies by region and by grade (regular versus premium), so cost per mile depends on both MPG and the price paid per gallon, not MPG alone. A premium-fuel car with the same MPG as a regular-fuel car will generally cost more to drive.

Is liters-per-100km a better metric than MPG?

Many fuel economy researchers think so, since it scales linearly with actual fuel used, unlike MPG. A drop from 10 to 8 L/100km represents the same real fuel savings as a drop from 5 to 4 L/100km, which isn't true of equivalent-looking MPG changes.