How this instrument works
Fuel consumption expresses how much fuel a vehicle uses per unit of distance — gallons per mile — which is the mathematical inverse of the more familiar miles-per-gallon (mpg) figure most drivers already think in. This instrument computes both from the same two inputs: fuel used and distance driven, so you get the gal/mi figure alongside the mpg number you're used to seeing on a dashboard or window sticker.
Gallons-per-mile framing has a real advantage for some comparisons: because it is a rate of fuel used per distance rather than distance per fuel, differences at the low-efficiency end of the scale show up more clearly. Going from 10 mpg to 20 mpg saves far more fuel over a fixed distance than going from 40 mpg to 50 mpg, even though both are 10-mpg improvements — a pattern that is easy to miss looking only at mpg, but shows up naturally when you compare gallons-per-mile directly.
The EPA and Department of Energy's fueleconomy.gov defines fuel economy the same way this instrument does: distance driven divided by fuel consumed for the mpg figure, with fuel consumption as its exact reciprocal. Whichever framing is more useful for your purpose — the familiar mpg, or the more granular gallons-per-mile — this instrument gives you both from a single entry of your actual fuel and mileage numbers.
- Enter Fuel used (gallons) — the gallons actually consumed over the distance you're measuring, from a fill-up receipt or trip computer.
- Enter Distance driven (miles) — the miles covered on that same tank or measured stretch.
- Read Fuel consumption (gal/mi) for the gallons-per-mile rate — useful for comparing efficiency improvements at the low-mpg end of the scale.
- Read Fuel economy (mpg) for the more familiar miles-per-gallon figure, computed as the exact reciprocal of the gal/mi rate.
- Use fill-up-to-fill-up totals rather than a single short trip for a more representative average, since short trips can skew fuel economy readings.
Worked example — 12 gallons over 360 miles
A driver fills up, then drives 360 miles before the tank needs 12 gallons to refill. Entering 12 for fuel used and 360 for distance gives fuel consumption = 12 / 360 = 0.033333 gal/mi, and the equivalent fuel economy = 360 / 12 = 30.0 mpg exactly.
Both numbers describe the same efficiency from two angles: this vehicle burns about 1/30th of a gallon for every mile driven, or equivalently, travels 30 miles on each gallon. Either figure can be used to estimate fuel needs for a future trip of known length in the same vehicle under similar driving conditions.
Questions
Why show gallons per mile at all when everyone already thinks in mpg?
Because gallons-per-mile scales linearly with actual fuel savings while mpg does not. Improving from 10 mpg to 20 mpg cuts fuel use over a fixed distance in half, but improving from 40 mpg to 50 mpg — the same 10-mpg jump — only cuts fuel use by a fifth. Comparing gallons-per-mile directly makes those real savings differences visible immediately, which is why some fuel-economy researchers and regulators prefer gallons-per-100-miles style metrics for comparing vehicles.
How do I calculate my own fuel consumption from a fill-up?
Fill the tank completely, zero your trip odometer (or note the current reading), drive normally until your next fill-up, and record both the gallons needed to refill and the miles shown on the trip odometer since the last fill. Enter those two figures — gallons used and miles driven — into this instrument to get both your gal/mi consumption rate and equivalent mpg for that tank.
Is fuel consumption (gal/mi) the same everywhere, or just a US measure?
Gallons per mile and miles per gallon are both commonly used in the US and a handful of other countries that use gallons and miles. Most of the rest of the world instead reports fuel consumption in litres per 100 kilometres, which is conceptually the same idea — fuel used per unit of distance — just built on metric units. Both approaches share the same underlying logic of relating fuel burned to distance covered.
Why might my calculated mpg differ from the EPA sticker rating on my car?
EPA window-sticker ratings come from standardized lab dynamometer tests meant to represent typical combined driving, but your actual mpg depends on your specific driving style, terrain, weather, vehicle load, tire pressure and how much city versus highway driving you do. It's normal and expected for a real fill-up-to-fill-up calculation to land somewhat above or below the official rating; tracking several tanks and averaging gives a more representative real-world figure than any single fill-up.
Do fuel consumption and mpg always give consistent results with each other?
Yes, always — they are exact mathematical reciprocals of the same two numbers (fuel used and distance), so gal/mi multiplied by mpg will always equal 1 by construction. If you compute consumption as fuel used divided by distance and mpg as distance divided by fuel used from the same fill-up data, the two will always agree; any apparent mismatch would only come from using different fuel or distance figures for each calculation.