You may think a car with a good crash rating is safe, but that rating might only apply against similar cars. Some crash ratings—most notably front crash ratings—cannot be used to compare cars in different classes due to weight differences. The Insurance Institute for Highway Safety (IIHS) notes that a larger vehicle that weighs more “provides better crash protection than a smaller, lighter one, assuming no other differences.” In tests run by the IIHS in 2019, two smaller vehicles with good crash ratings performed poorly in crash tests against midsize and large cars.

If vehicle safety is one of your priorities when you purchase a new vehicle, the ratings provided by the IIHS and the National Highway Traffic Safety Administration (NHTSA) are helpful. However, it’s important to understand their limits.