I think the only differences between the engine/wheels on the 3.3 vs the 2.5 are that the 3.3's came with a wide ratio output gear in the trans and the extended front axle. The revo I had my 3.3 in had the longer chassis, standard gear ratio, so don't know why it was such a dog. Even with the 21TM in it, it's not overly wheelie happy, but it does seem to be much less labored. I was running 38/15 gearing with the 3.3 and still am with the OS21TM.