might sound like a silly question but does a r rd measure from the wheels or the flywheel???????

 

both, i think

 

The wheels. The 'flywheel' figure is a best guess, at best. The 'wheels' figure isnt that reliable either, being massively affected by other factors: transmission temperature and tyre pressure to name but two...

 

both but as simon mentioned the 2 variables when i've had mine done the tyres were way over standard road pressures.

 

Also just to add - after measuring the power at the wheels some use the run down to estimate the transmission losses to arrive at the flywheel figure.

 

They do, but it is little more than a best guess...

 

Provided the wheels don't slip why would tyre pressure make a difference?

 

Because the lower pressure they are, the more rolling resistance they have, soft tyres reduce the at the wheel numbers.

A rolling road can only measure at the wheels numbers, from that flywheel figures can be obtained my a number of methods...
1/ %age guess - pretty poor and out of date these days
2/ Prior experience, test a number of stock cars, assume they produce the stock power curve and create a loss V speed table.
3/ measuring loss by driving the car (in neutral) the problem is that friction in transmission changes with load due to the side loads on the bearings, shafts bending slightly etc.

Simon