If you download the AV ( on the Realflight Swap Pages ) that I posted for my Big Stick 60 using a SuperTiger 75 engine, you'll find that it produces EXACTLY the same flight speed and power as the real thing.

The sim CAN be INCREDIBLY accurate, but all to often the engine specs reflect what the manufacturers STATE, and not what we see in the real world... hence what Jester_s1 says is right on the money, yet the sim is otherwise extremely accurate.

The Physics model I posted adjusts for differences. The actual airfame model only needed a small amount of tweaking to produce real world results.