No longer rumor, it’s been established that 60s Muscle Car HP was under-rated. We explain how and why and how much horsepower these motors actually produced.
There have been two reasons given for the motivation of the factory to under-report the horsepower of their muscle cars: insurance and NHRA classifications.
Certainly insurers were getting nervous when they started to see cars being introduced, starting with the Pontiac Tempest GTO, with high horsepower in a relatively small car. And as all insurance is based on risk, the insurance companies apportioned higher premiums on these big horsepower cars. To what degree the insurance companies saw through this rouse is unknown. The under-rating rumors were all over the car magazines at the time, and insurance companies aren’t dumb, so in the end it may not have made much difference.
The other reason is much more tangible. The NHRA placed new cars into Stock categories based on weight and stated horsepower. And in the 1960s, drag racing was as important to car sales as NASCAR is today. At that time, Indy was a one-time a year event, ther other events not receiving much coverage, and NASCAR was a regional series in the Southeastern US. Drag racing was where it was at – just a quick survey of music (409, Little Deuce Coupe, Little Old Lady from Pasadena, etc.) and TV shows – Grandpa Munster had a dragster and a drag-style custom car even appeared on Star Trek. As just a wild guess, it was probably 75% NHRA, 25% insurance as motivation for under-rating an engine.