at 55 you are knocking on 3000 rpm. as long as you have good oil pressure and not running hot I would not consider it detrimental to the engine, just annoying. Depending on what I am pulling I can usually hit 50 mph in 4 high with the 8.11:5.83 rear axle. according to my tach that is 2600 rpm and I hold that for 30 miles at a time on some trips. If I am pulling one of my smaller tractors I run around 2800-3000 rpm. at 55 mph. I was able to pull a 56 ford 600 series tractor on a 25 ft lowboy gooseneck at 55 mph at 2000rpm in OD. of course in south LA we have no hills and it was not real windy. The wind is a killer on topend lol