Horsepower Calculator

Power from torque and engine speed.

The EPA rating on an American window sticker has been adjusted twice since 2008 to reflect faster highway speeds and air-conditioning use, so older figures read optimistically. Electric vehicles invert the comparison entirely, quoting miles per kilowatt-hour.

American fuel economy is quoted in miles per gallon, where higher is better — the opposite direction from the litres per 100 km used elsewhere. Pump prices vary widely by state because fuel taxes do, and the annual cost of driving is distance divided by mpg, multiplied by the price of a gallon.

Travel cost on the road is not the same as the sticker price of the trip: tolls, parking and tyre wear sit outside a fuel figure.

Cost per mile is the figure that makes two cars comparable, because it folds in fuel, depreciation, insurance and maintenance rather than fuel alone. Speed dominates fuel use: aerodynamic drag rises with the square of velocity, so the difference between 65 and 80 mph is substantial.

Horsepower Calculator sits with the rest of the library's everyday work, and it rests on the same driving-cost arithmetic the way a manufacturer's label or a timetable does. Its answer is only as good as the assumptions standing behind it.

The quantities it works from — torque and engine speed — are the ones a real driving-cost question actually turns on, and each carries a convention of its own, set out below.

Everyday arithmetic in the United States carries conventions that are easy to miss: dates run month before day, distances and weights follow customary units, and prices are quoted before tax. Those conventions make a surprising share of daily sums come out differently from the same question asked elsewhere.

The figures in this section follow American practice unless the subject itself is international.