[Silverback51]

The majority of engine braking is a result of the carb butterflys being closed and you have turned the engine into a hugh vacuum pump. Diesels have far greater rotating mass and they require exhaust or engine brakes to assist in braking. In reality, the greater mass of a heaver flywheel will require more braking than a lighter flywheel.

Different tires can have a considerable affect on rolling resistance. Brake pad material can also have an effect. The fluid weights in the transmission, and rear end can also be a factor.

Then you can factor in the clearances in the engine, and the rest of the running gear.

Noticing a trend here? There are so many factors that can affect rolling resistance, that unless it's an absolute twin of the other car, right down to the tire pressure, there will be differences.