Some rolling road read the wheel power during a power run and then read the coast down drag factor, the drag factor is then added to the wheel power to give calculated fly wheel power.
Then some rolling roads can only read wheel power, and then dyno dynamics rolling roads can only read the wheel power during a power run and had no way of reading the actual losses, in the states most dyno dynamics operators will only give the reading as wheel power, but most of the uk dyno dynamics rolling roads will give a calculated estimated flywheel power based on a fix % which is worked out in the software. A rule of thumb is a front wheel drive car will have around 12% losses on average, and read wheel drive 22-25%. This is why there is a huge dyno lottery between all rolling roads, not one dyno is 100% correct, but if used correct all dynos inc the calculated loss dyno dynamics road will give realistic reading for the condition of the run on a given day.
Only way to get an exact flywheel bhp is take the engine out and run it on a bench dyno
