Gas Mileage Calculator

This calculator measures observed fuel economy rather than predicting it, so the quality of the answer depends on how the two inputs were collected. The standard tank-to-tank method is to fill the tank to the pump's automatic shut-off, reset the trip odometer, drive normally, then refill to shut-off and record both the trip distance and the gallons it took to refill. The gallons figure must be the quantity added at the second fill, not the tank's rated capacity. A single tank is typically accurate to only 5 to 10 percent, because the shut-off point shifts with nozzle position, pump flow rate, and forecourt slope, and because odometers commonly read within 2 to 4 percent rather than exactly. Averaging three to five consecutive tanks removes most of that noise. Interpreting the result needs a reference point. New light-duty vehicles sold in the US average in the high twenties MPG combined, while the on-road fleet including older vehicles sits closer to 22 to 25. The metric equivalent is a reciprocal, not a rescaling: litres per 100 kilometres equals 235.215 divided by MPG, and fuel per distance is the more useful form for judging savings. Research published in Science in 2008 by Richard Larrick and Jack Soll showed that expressing efficiency as distance per unit fuel leads people to systematically underestimate the benefit of improving inefficient vehicles, an effect they named the MPG illusion, and it is why US window stickers now also show gallons per 100 miles. The annual projection here assumes a fixed 12,000 miles and a constant fuel price; typical US annual mileage runs nearer 13,000 to 14,000, so scale accordingly.

Standardised fuel economy figures in the United States are a by-product of emissions regulation. When the Environmental Protection Agency began laboratory testing vehicles for exhaust emissions, the same dynamometer runs could be used to compute fuel consumption from the carbon in the exhaust, and the agency started publishing comparative fuel economy data for the mid-1970s model years using its city cycle and a separate highway cycle. The Energy Policy and Conservation Act of 1975, passed in the aftermath of the 1973 oil crisis, turned those measurements into policy by creating the Corporate Average Fuel Economy standards, which set fleet-wide targets manufacturers had to meet and made a comparable, official MPG number a legal necessity rather than a marketing claim. The published numbers have been revised downward twice because owners kept observing lower figures than the label promised. In 1985 the EPA began applying adjustment factors to its raw laboratory results, cutting city estimates by about 10 percent and highway estimates by about 22 percent. A larger change followed in 2006, when the agency adopted a five-cycle procedure effective with the 2008 model year, adding tests for cold-temperature operation, higher speeds with more aggressive acceleration, and air-conditioning load, which lowered posted combined ratings again by roughly 10 to 15 percent for many vehicles. The redesigned fuel economy label introduced for the 2013 model year added gallons per 100 miles, an estimated annual fuel cost, and a five-year cost comparison against an average vehicle, reflecting the same shift toward cost-per-distance thinking that this calculator reports alongside raw MPG.