I have a 2003 Rebel 250 and on the freeway when it’s flat I’ve had it over 70-75 but hills will kill it in 5th gear. A few months ago I took a 25 mile trip on the freeway which had hills and going up the hills I was maxed out between 60 and 65, just moved over to the right hand lane and got in line behind all the trucks. I don’t know how much you weigh but the weight makes a big difference on the small bikes (I’m 190+). Another thing to keep in mind is that the Rebel doesn’t have a windshield so your body if you are sitting up straight is acting like a big wall. To test this lean down a little like you are a riding a sport bike and watch your speedo climb.
The thing about the Rebel is 5th gear has very little tolerance to be able to hold and accelerate up hills. If I’m accelerating up to freeway speed I shift late, taking it almost to those little red lines on the speedo (that’s the engines redline), that’s where the Rebel makes most of it’s torque it’s a high reving engine. From a post on the Rebel forum I found the following “the powerband to be between 15 and 35 mph in 2nd, 35 and 50 in 3rd, 50 and 65 in fourth, and the rest in 5th”. That pretty much matches what I’ve found.
Of course at those revs it’s going to be like an electric toothbrush for your feet. All those things made me decide that it’s not my first choice as a bike for the freeway but I’m so glad I got it to learn on, and it’s a hoot around town and at slower speeds. For me the Rebel is happest cruising along at 35-55 mph.