Not shifting into overdrive?

My car runs great, except 1 thing ive noticed that I hope its a real problem, while on the interstate where the speed limit is 70, I run it 70-80 and the RPMs run 2500-3200. It sounds like its running high like it needs to switch another gear. Ive yet to think to listen to the gear changing to see if its missing top gear, but is that possible, but if its actually in its top gear doing that, is that bad, can that cause any more problems and/or how can that be fixed? I do travel allot. 2hrs to 6hr drives one way atleast twice a month. Thanks.