A late model 460 (curve below) with 4.10 gears and an E40D and standard tires runs 65mph at 2,000rpm. He's capable of making 395ft# at that 2,000rpm so he's running with the cruise on and doesn't even notice the hills--he has 180hp available to him at 2,000rpm.
A 300 with the same set-up running at 2,000rpm has 255ft# available and only 97hp available at that rpm.
So both trucks hit the hill that requires 148hp to maintain speed up. The 460 guy is talking to his wife on the cellphone--the cruise pushes the throttle open, the engine has has 180hp available, more than enough to spare and goes up the hill in OD.
The 300 guy only has 97hp available in OD and his truck starts to slow. He downshifts, the rpms jump from 2,000 to 2,800. He's now got 120hp available but he's still slowing. He goes down to 2nd. Unfortunately, his engine wants to shoot up to 4300rpm--he's between gears here. So he has to settle with going back up to third and letting the truck slow down to the speed 120hp will maintain on the hill. On the downhills he's running 75 to catch back up to the 460. And all that shifting kills fuel economy and massively heats auto-trannies (if applicable).
So the 300 guy wants to regear to try and make his truck keep up. To get the most from his engine he needs to make it do 3400rpm at 65 on that hill. He can go from 4.10s to 3.08s and plan to downshift to 2nd on that hill--plus he'll be running 3rd on lots of other lesser hill, and his take-off from a stop will be greatly impaird. Or he can go from the 4.10 to a 5.13, downshift only once on that big hill, have better take-off from a stop but then he's running 2500rpm at 65 and will get reduction in his fuel economy on the stretches. He's probably going to go with the 5.13 though.
So see what you are really faced with before just arbitrarily changing the gears. They can help a good deal if you are willing to accept the day to day differences. Or maybe you don't mind going slow on hills.
I did an 800 mile round trip in my 450ft# turbodiesel with Sam in his 315ft# 5.8 F250--identical trailers and loads. It was just as I described above--I left him on the hills and he came barreling up from behind on the down hills. He got 7mpg to my 16mpg and hated my guts for it. The fact he ended up with a Powerstroke after that should tell you something. It's a lot better trip if you have a lot more power available. I used horsepower in this thread because it really is the important yardstick--it's the rate work is done and pulling a trailer is work. The formula is [HP= (TQ * RPM)/5252]