Depends... on... gearbox, gear ratio, diff type, diff ratio, engine speed... If you're talking about losses between the flywheel and the *road* then it's also dependent on tyre size/type/pressure as well (not to mention the road surface). There is no percentage figure that 'works' for all conditions. A further point to note is that losses on a rolling road tend to be higher than on a (real) road due to:
* The rollers aren't flat, and there are 2 of them resulting in significantly lossy distortion of the tyre
* Powerful cars have to be strapped down hard onto the rollers, resulting in even more of the above.
* More powerful cars have to be strapped down hard and have 1-3 fat bastards sat on the back leading to even more of the above
* Sticky stuff squirted onto the tyres to help them grip the rollers
As a rule of thumb say 15-25% for all of the losses (the high end of that on a rolling road), of which a significant proportion is in the tyres themselves.
Fun eh?
Mike