But this doesn't let the fanboys down to keep talking about this sim. My point is, it can feel like the starship enterprise on three triangle shaped wheels but kids still going to support / race it because they identify with cars they know and 'have seen' and are stunting on real life tracks. Nobody is interested in 100% real match, not even 20%, looks and brands thats the only thing that counts in 2013/14.
Using some telemetry you could easily see where the differences come from. Does ac have any kind of telemetry like motec, atlas or replay analyzer?
Longitudinal acceleration vs engine revs should tell exactly where the differences in the engines are if you do a 50-120kmh run using just 4th gear in both sims. Or if it is aero issue or tire pressures or some other wonky stuff which probably shows up if you do a 100kmh-50kmh run coasting on 4th gear.
Did you have same tire dimensions for both cars? I take it that you had flat piece of track for both tests.