tire size & gear ratio

I`m running 255/60-15`s that are 27.5 in. tall with 3.91 gears and a 4spd.. My bb at 60 mph is about 3200 rpm. I`m happy with the acceleration but not the cruising. I`m thinking of going with a 5spd. trans so I can do some cruising without sacrificing the acceleration. It`s a bit of a balancing act to try and get the whole package.