My 300tdi 110 currently runs at 3000 rpm at 55mph and about 3500 at 65 in 5th gear . Vdo rpm gauge, 22d lt230 and 31 inch tires. Speed from gps. According to Ashcroft calculator I'm pretty high, although it does pull very well. Thinking it's been regeared but is there any way of telling without pulling it apart? I would like a little lower rpm for the long interstate drives. And is 3500 harmful over an extended time?