Just bought new John Deere JX75 and used it Sunday for the first time. Great mower and great cut. The dealer says the engine is tuned to 3,400 RPM. At tjhis setting, 4th gear feels too slow and 5th gear a little too fast. Anyone have any experience increasing the RPMs to 3,500 or 3,600 to increase speed of gears. That way I can mow in 4th gear and use 5th gear for transport? According to the manual, the engine can be operated betwn 3,200 and 3,600 RPMs. Any downside to increasing the RPMs?