When going 40 mph and keeping foot steady at that speed the trucks RPM"S will go up 3000 rpm's for a couple of seconds then back to where it was and do it a few times while going down the road? Doesn't seem to do it at 55 , 60 , 70 etc?
Almost like it's going to shift into a passing gear?
Is this the little fuel pump or tranny or what?
thanks

 

sounds like your tranny is looking for the rpm sweet spot between gears. I wouldn't worry about it unless the thing starts slipping.

 

Rootercycles , thanks for the reply!!!
It's what a diesel mechanic firend said today also , so i feel good !