Horsepower ratings are messy

Horsepower sounds like one number, but carmakers have long mixed different tests into the same badge. A “300-horsepower” car may be quoting gross, net, peak, or 30-minute output, depending on the era and the powertrain. (jalopnik.com) For gasoline cars, the biggest old split was “gross” versus “net.” Gross measured an engine on a stand with fewer real-world losses, while net measured it with production parts like the air cleaner and exhaust attached. (sae.org) That change hit U.S. buyers in the early 1970s. General Motors and Chrysler showed both figures in 1971, and net ratings became the industry norm for 1972, making some muscle cars appear to lose more than 100 horsepower on paper. (hagerty.com) California helped force the switch. State law says any advertisement, brochure, owner’s manual, or sales manual for a gasoline vehicle under 6,000 pounds from the 1972 model year onward must use the Society of Automotive Engineers “as installed” net horsepower rating. (law.justia.com) The Society of Automotive Engineers still updates that net-power rule. Its revised J1349 standard, released on April 6, 2026, says engines must be tested with production components such as air cleaners and exhaust systems so published numbers better match real-world installation. (sae.org) Electric vehicles added a new layer of confusion because regulators and marketers often talk about different kinds of output. United Nations Economic Commission for Europe Regulation No. 85 defines both “net power” and “maximum 30 minutes power” for electric drive trains, treating sustained output as a separate figure. (eur-lex.europa.eu) That means an electric car can post a high peak number for a launch or a short burst, then deliver less power once heat and battery limits build up. Jalopnik’s explainer says that gap is where many modern horsepower claims become hard for shoppers to compare. (jalopnik.com) Tesla has used both terms in its own Model 3 Performance messaging. Coverage of the April 2024 update said Tesla claimed a 22 percent gain in continuous power and a 32 percent gain in peak power, showing that the two figures can move by different amounts in the same car. (motortrend.com) Even certification does not cover every number consumers see in ads. Society of Automotive Engineers J2723 governs certification of engine power and torque to J1349 or J1995, but that program is aimed at engine ratings, not every combined hybrid or electric headline figure used in marketing. (sae.org; sae.org) So the safest question is not “How much horsepower does it make?” but “Under which test, and for how long?” Until labels answer both parts, the number on the brochure will keep saying less than buyers think it does. (jalopnik.com; eur-lex.europa.eu)