When I set up 40 size trainers, it's always an OS 55ax. It's not for the speed, it's nice to have the power to undo what the student just handed you. Then again, I do wait to the last moment to take it, I want the student to not give up and fight to the last. Students love it. Haven't lost a trainer The fuel usage is lower than any 40-60 engine I've ever come across, air time at training speeds is unreal.