Fuel Cost Calculator

Trip fuel cost is the product of three quantities, but two of them are expressed in mutually inverted conventions and that is where most errors come from. Miles per gallon is distance per unit of fuel, so fuel used is distance divided by efficiency. Litres per 100 kilometres is fuel per unit of distance, so fuel used is distance multiplied by efficiency and divided by 100. The two scales convert through the constant 235.215, meaning 30 MPG is about 7.84 L/100km, and because the relationship is reciprocal rather than linear, equal-sized gains in MPG do not represent equal fuel savings. Moving from 15 to 20 MPG over 10,000 miles saves about 167 gallons, while moving from 35 to 40 MPG over the same distance saves only about 36. Cost per mile is the more useful comparison figure: at 3.50 dollars per gallon and 28 MPG it works out to 12.5 cents. The efficiency figure entered is the dominant uncertainty. Published ratings come from standardised test cycles and real-world consumption commonly runs 10 to 25 percent worse, with the gap widening in cold weather, in stop-start traffic, and when towing. Aerodynamic drag rises with the square of speed, so cruising at 75 mph rather than 65 typically costs 10 to 15 percent more fuel. A tank-to-tank average over several fill-ups is a far better input than a manufacturer figure. This is also a fuel-only figure: once depreciation, insurance, tyres, and maintenance are counted, the US Internal Revenue Service standard mileage rate sits near 70 cents per mile rather than near 13. The calculator does not convert currencies, so price and result share whatever currency you enter.

The price side of this calculation has been shaped less by markets than by tax policy. Oregon enacted the first US state gasoline tax in 1919 at one cent per gallon, an attractive levy because it fell in rough proportion to road use, and every state had adopted one by 1932. The federal government followed with a one cent per gallon excise in the Revenue Act of 1932, initially a general deficit measure. The Federal-Aid Highway Act of 1956 dedicated that revenue to the Highway Trust Fund and tied fuel pricing directly to road construction. The federal rate was last raised in 1993, to 18.4 cents per gallon for gasoline, and because it is not indexed to inflation its real value has fallen substantially since. Retail price volatility entered public consciousness with the 1973 OPEC embargo, which quadrupled crude prices within months and produced US rationing queues, a national 55 mph speed limit enacted in 1974 explicitly as a fuel conservation measure, and the first federal fuel economy standards. A second shock followed the Iranian revolution in 1979, and US price and allocation controls were lifted in 1981. The habit of checking a current per-gallon price before a long drive dates from this period, supported today by the weekly retail gasoline price series compiled by the US Energy Information Administration and by daily national averages from motoring organisations. European practice diverged in both units and magnitude, pricing per litre with duty and value-added tax making up well over half of the pump price in many countries.