Engine rpm problem

i have a 05 hummer sut. when driving at a constant speed between 45-55mph the engine rpm’s ruv up and down a bit. any help will do

Is it possible you are going up and down hills and it is kicking between overdrive and the next lower gear? if so (not familiar with hummers) you might avoid the problem by using a different position on the gear selector, ie d in square vs d3?

just regular flat road driving. no stress on vehicle.

The I5 engine is overmatched by the weight of the truck. I’m guessing it doesn’t have enough power to keep the truck moving in OD at 45-55 MPH and has to downshift to maintain speed. I’ve seen this happen on a number of underpowered vehicles. The 91-95 Suburban with the wheeze old TBI 350 comes to mind as well.

it is a 05 h2 sut with a 6ltr engine

I think that the shifting thing is plausible. If the truck has one, hit the O/D Off button and give that a whirl. If not, do as waterboy suggested and go down one slot below D on the shifter (e.g. to 3 or if you have a circled D and a plain D go with the plain D). Then see what happens.

my mistake I thought you had the H3 SUT.