no rolling road is spot on , that i assure you , and its a mith that cars produce over there stated bhp .....yes maybe on an engine dyno they give 3 or 4 bhp over . tolerances on modern engine prodiction is much better these days and they are always within a bit , having experience of many rolling roads with many different cars i can tell you everyone always says theres is spot on .
i should have brought a car up thats been on an engine dyno , that way its a known power output , we have doen this in the past and not 1 has given what the dyno does , however the closest came from a set of rollers that clamp to the hubs .
and tyre presures up or down can affect the results , try it net time you do a power run , deflate the tyres and watch the results change.
i am sure you will have a slightly different output but these claims of 6bhp for an aitfilter and 9 bhp from a backbox are not true , don't ask a VAG tech ask and engine builder to explain it to you.