Take the spark plugs out and take a close up picture of them, post it here and we can tell you if it is lean.
If so you need to see what jets are in it and go 2-3 steps richer and try it.
I had a car doing the same, didn't think it was lean because it ran so well, smooth transitions and all. It would over heat at 55 mph!! richer jets cured it. Power went up and so did mpg.