I have 3.55 gearing in my 2.5 and I'd swapped in an AX-5. I really only use 5th on the interstate, I can occasionally get away with it at 65, but on on flat roads. If there's any hills, I don't use 5th whatsoever. Remember, lower RPMs does NOT mean better mileage, if the engine has trouble turning the gear, it can actually make it worse. Also, I've sat around 3400 RPM for 4-5 hours pretty much straight with my 2.5 and it doesn't hurt anything. It may sound like it's about to explode, but it's fine.
Another thing to consider is my 2.5 is a 91, therefore I have the MPFI. IIRC, the MPFI 2.5s are pushing 15-20 HP over a TBI 2.5. She's also sitting on 205/75 R15 tires, which I believe are the smallest from factory.