Had to go back and look at my original post, Sorry my bad, must have been replying while
boozing. I don’t know why I was thinking rear wheel horsepower. Here are the definitions as I understand them. As a general rule, rear-wheel horsepower on a manual-transmission car is about 15% less than SAE net, and rear-wheel horsepower on an automatic-transmission car is about 20% less than SAE net.
SAE Gross Horsepower
This is the old process that manufacturers used as a guide for rating their cars. It was in place until 1971. SAE gross also measures horsepower at the flywheel, but with no accessories to bog it down. This is the bare engine with nothing but the essentials attached to it; little more than a carb, fuel pump, oil pump, and water pump. It is impossible to provide a mathematical calculation between SAE net and SAE gross. As a general rule, however, SAE net tends to be approximately 80% of the value of SAE gross. SAE J245 and J1995 define this measurement.
SAE Net Horspower
In 1971, manufacturers phased in SAE net horsepower. This rating is measured at the flywheel, on an engine dyno, but the engine is tested with all accessories installed, including a full exhaust system, all pumps, the alternator, the starter, and emissions controls. Both SAE net and SAE gross horsepower test procedures are documented in Society of Automotive Engineers standard J1349.