A car out the factory is at the fly

A car on the rolling road is at the wheels and flywheel can be calculated

The most accurate is a bench dyno where its just the engine sat on a bench.... with a dyno

There is a couple of different rolling roads as well
Electromagnetic, sits in 2 rollers and an electromagnet creates resistance, can start to become inaccurate after a number of runs, as the electromagnet gets hot and doesn't create as much resistance as its meant to

An inertia one is a single drum that you balance on, its weighted already and is more accurate
They have one at track n road in essex, its a certain one cant remember exactly but 1 of only 4 in the world and about as accurate as you can get


As explained to me anyways.....