What rpm at 55? What rpm at 75? Does it have a cat? True dual exhaust all the way back? I assume fuel injected. I don't doubt the numbers you list. Could just be that you are too low on the torque curve at 55. Engines can be tuned for max efficiency at a specific rpm. They do this with stationary engines like generators that need to run for a loonngg time at a very specific rpm with very little variation.