It is not that easy George
You only use a back to back comparison on that car at that time
That is all rolling roads are designed for. I used one for 7 years, an electronic one calibrated every 6 months when we did re-map work and performance mods
I also found lots of discrepencies you would not think of, like moving the crown wheel to drive on the leading edge of the teeth rather than the trailing edge, even that made 1.9 BHP difference
All different weather conditions move the figures- Wet miserable day- heavier air through the intake- more power at the wheels. Tomorrow hot sunny day less power at the wheels.
 You don't stop playing because you get old, You get old because you stop playing.

07 TDV6 been and gone. 08 TDV6 been and gone. 08.5 TDV8 in evaluation mode and now gone. BMW 535D M sport Been and gone, Now time for RRS No4 - TDV6 HSE in Stornaway with Ebony - 09 Model 6 months old.
RS Cosworth 4x4 Stage 2 Mint for summer
Audi TT
Astra VXR for the daughter.

Thanks for info the Nige,

Also, I've just read that automatic gearboxes can cause problems with rolling roads and that it's impossible to get totally accurate readings when using a vehicle with auto transmission? "People simply disappeared, always during the night. Your name was removed from the registers, every record of everything you had ever done was wiped out, your one-time existence was denied and then forgotten. You were abolished, annihilated: vaporized was the usual word."

Hi George
That is correct. The torque convertor etc all have to be allowed for
When you drive the manual in the gear that gives as near to 1:1 ratio at the wheels that is as accurate as it can be.
When you go for full power you go full throttle so on an auto you need to implement a system that stops the kick down situation for your power reading.
There are lots of for and against the biggest being when joe public hears the words "rolling road" they think you can simulate road conditions which obviously you never can.
They are an exellent piece of kit as long as you know the limits. We only had this as part of our development so although it was eventually opened to the public it didn't have to make a profit. When Vauxhall had the sport division-Irmscher UK in the early 90's they bolted their exhausts, camshafts etc on and then we re-mapped the ECU and raised the rev limiter etc to suit the engine. Saloon race car development for Honda UK removing the speed limiter from the ECU on the V-Tec engine, changing the point at which the V-Tec kicks in etc, all these type of jobs you need it for but normally road cars you don't.
The loss between the Flywheel and wheels can go from anywhere between 15% and 30% depending what type of transmission (straight cut gears on race cars etc) 2WD 4WD- they are all different. You don't stop playing because you get old, You get old because you stop playing.

07 TDV6 been and gone. 08 TDV6 been and gone. 08.5 TDV8 in evaluation mode and now gone. BMW 535D M sport Been and gone, Now time for RRS No4 - TDV6 HSE in Stornaway with Ebony - 09 Model 6 months old.
RS Cosworth 4x4 Stage 2 Mint for summer
Audi TT
Astra VXR for the daughter.

Nige,

Thanks for the info  "People simply disappeared, always during the night. Your name was removed from the registers, every record of everything you had ever done was wiped out, your one-time existence was denied and then forgotten. You were abolished, annihilated: vaporized was the usual word."