Miles per gallon is the standard fuel-economy figure in the United States, and its formula is the simplest in this guide: miles driven divided by gallons used. The subtleties are not in the formula but in the scale: MPG is reciprocal, so equal steps in MPG are not equal steps in fuel saved; in the conversion to litres per 100 kilometres, where the scale runs the other way; and in cost per mile, which is the figure that actually prices a trip. This guide works all three, with the measurement habits that keep the formula honest.

The formula, and how to measure it

Fill the tank completely and record the odometer reading, or reset the trip meter. Drive normally until the tank is low, refill completely, and record the gallons added and the miles driven. MPG is miles divided by gallons: 300 miles on 12 gallons is 25 MPG. The refill must be complete in both cases, because the formula measures fuel used by fuel replaced, and a partial fill breaks the equality.

One tank gives a noisy figure: pump shut-off variation, a different pump, a slope at the station, all move it by a few percent. Three or more tanks averaged give a reliable figure, and the calculator on this page averages across fills for exactly this reason. A single-tank figure that disagrees with the dashboard by a few percent is measurement noise; a persistent disagreement across tanks is a vehicle issue worth investigating.

The reciprocal scale: why the bottom matters most

Fuel used is miles divided by MPG, so fuel saved by an improvement depends on where on the scale it happens. Over 12,000 miles a year, improving from 15 to 20 MPG saves 200 gallons: 800 down to 600. Improving from 30 to 40 MPG saves 100 gallons: 400 down to 300. The second improvement is twice as large in MPG and half as large in fuel, because the scale is reciprocal and the bottom of it is steep.

The buying implication is direct: when choosing between thirsty vehicles, a small MPG gap matters enormously, and when choosing between efficient ones, a large MPG gap matters less than it looks. A buyer who pays a premium for 40 MPG over 35 MPG saves about 43 gallons a year over 12,000 miles — real money, but far less than the premium for 20 MPG over 15 MPG, which saves 200 gallons.

L/100 km: the scale that runs the other way

Most of the rest of the world uses litres per 100 kilometres, where a lower number is better — the opposite direction from MPG, where higher is better. The conversion is: L/100 km equals 235.215 divided by MPG. So 30 MPG is 7.84 litres per 100 km, 40 MPG is 5.88, and 20 MPG is 11.76. The constant 235.215 combines the gallon-to-litre conversion, 3.78541, with the mile-to-kilometre conversion, 1.60934, in one figure.

The reciprocal relationship means MPG comparisons and L/100 km comparisons agree on ordering but not on gaps: a vehicle that looks much better in MPG may look only slightly better in L/100 km, or the reverse, depending on where on the scale the comparison sits. Compare in the unit that prices the decision — gallons for fuel bought by the gallon, litres for fuel bought by the litre — and convert only for communication.

Cost per mile and trip cost

Cost per mile is fuel price divided by MPG. At $3.50 a gallon and 25 MPG, a mile costs 14 cents in fuel, so a 300-mile trip costs $42. At the same price and 35 MPG, a mile costs 10 cents and the trip costs $30. The same trip, the same fuel, and a $12 difference that MPG alone does not state plainly — which is why cost per mile, not MPG, is the figure to use when pricing travel.

Fuel price is a user-entered figure in the calculator because it moves daily, and any page that printed a live price would be wrong by tomorrow. Enter the price you actually pay, at the station you actually use, and the trip figure is honest. A cheaper gallon at a distant station is only a saving if the detour costs less than the saving: 10 cents a gallon on 12 gallons saves $1.20, while a 10-mile detour at 14 cents a mile costs $1.40.

Improving fuel economy, honestly

The honest improvements are unglamorous: correct tyre pressure, which reduces rolling resistance; removing roof loads and excess weight; smooth acceleration and steady speeds, which keep the engine in its efficient range; and regular maintenance, especially air filters and spark plugs on older vehicles. Each is worth a few percent, and together they are worth more than most gadgets.

The dishonest improvements are the ones that change the measurement rather than the economy: overfilling past pump shut-off, which corrupts the next fill’s figure; drafting close behind trucks, which saves fuel and risks a collision; and coasting in neutral downhill, which saves little on modern fuel-injected engines and surrenders engine braking. Measure honestly, improve mechanically, and let the averaged MPG show the result.

Frequently asked questions

How is MPG calculated?

Miles driven divided by gallons used. Fill completely, record miles, refill completely, and divide. Average across three or more tanks for a reliable figure.

How do I convert MPG to L/100 km?

Divide 235.215 by the MPG figure. So 30 MPG is 7.84 litres per 100 km and 40 MPG is 5.88. Note the scales run in opposite directions: higher MPG is better, lower L/100 km is better.

Why does 15 to 20 MPG save more fuel than 30 to 40 MPG?

Because MPG is reciprocal: fuel used is miles divided by MPG. Over 12,000 miles, 15 to 20 MPG saves 200 gallons, while 30 to 40 MPG saves only 100 gallons.

How do I calculate cost per mile?

Divide the fuel price by MPG. At $3.50 a gallon and 25 MPG, a mile costs 14 cents in fuel, so a 300-mile trip costs $42. Enter your own fuel price, since prices change daily.

Why does my MPG differ from the dashboard figure?

A single tank is noisy: pump shut-off variation and station slope move it by a few percent. A persistent disagreement across several tanks is worth investigating as a vehicle issue.

Does tyre pressure affect MPG?

Yes. Under-inflated tyres increase rolling resistance and reduce economy by a few percent. Correct pressure is the cheapest fuel-economy improvement available.