The brilliant McLaren 720S Graham recently drove gets its name because the 4.0-liter twin-turbocharged V8 mounted behind the seats makes 720 horsepower. Clever. And impressive, except that, when you read the fine print, it actually makes "just" 710 horsepower. How’s that so?

It’s all too common to see a slight discrepancy in horsepower figures when comparing Europe to the United States. No, they don’t inflate the numbers, but they do calculate them differently. It’s because we use U.S. standard units and they use the metric system.

Horsepower is power, which is work over time. The term comes from James Watt, an engineer whose name resides on every lightbulb you purchase. He determined a horse can perform 33,000 foot-pounds of work in 1 minute. Divide by 60 seconds to get the more widely known (at least among engineers) 1 horsepower equals 550 foot-pounds per second. Europe uses the same horsepower term but replaces pounds with kilograms of force and feet with meters for a figure of 75 meter-kilograms of force per second. With a bunch of math, you can put U.S. and metric horsepower in the same units. It looks like this:

You don’t need to know all those calculations. Just know that German pferdestärke (PS) and Italian cavalli vapore (CV) translate to horsepower, but 1.4 percent less than the SAE horsepower at which cars are rated here.

What about kilowatts, another term you’ll see on occasion? 100 kilowatts equal 134 U.S. or 136 metric horsepower.

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