[JamesNoBrakes]

Originally Posted by angryBits

Purely based on numbers.

85% x 650 = 552.5 at the wheels.

85% drive train loss is a general guess but it seems to be about right.
it might be interesting to look at numbers for M6 vs A10 to see if theres much of a difference. Guessing not much if any, the A10 is very modern.

Yep, dependent on many things, hard to just pick a number, but it can get you started in the right direction. The WRX I used to own was supposed to be "under-rated", that was BS when you hooked them up to a dyno. What they did have was short gearing and a good kick in the pants from the turbo and AWD, which made them feel quite fast at lower speeds. They did pick up a bit of HP when they changed to the 2.5 block, but the HP ratings for all manufacturers were revised at the same time, so it wasn't apples to apples anyway.

You only start to get a good idea when you see a bunch of dynos posting similar numbers for similar circumstances.

I'd say that manufacturers for the most part do not under-rate because they realize it's a paper-game and that's how most people buy their car. Few people, if any, are going to unleash all the HP on a test drive and it'll come down to what each is "capable of". Those that are buying the higher end cars are more in the "know" and they are likely buying a BMW or Audi for reasons other than just the HP, so less of an effect there.