In the old days, we were always taught to tow in 4th gear, especially if 5th wasn't strong enough, both from an available power standpoint, plus 4th was the best gear since it wasn't a gear, it was a straight 1:1 with everything locked together and transmitting power straight through. The thing is, on my 2003 Honda Civic Hybrid, 4th gear is not a 1:1, but a slight overdrive, maybe 0.927:1; none of the gears are 1:1.
On my Civic towing a small teardrop camper, 5th gear doesn't have the power to overcome wind resistance at 60mph at 2,000 RPM. The engine functions in constant electric assist mode until the battery drains then the speed begins a terminal plunge since the engine alone doesn't have the power to pull the parachute er i mean camper at speed and once the RPMs drop low enough (getting close to 1500 at 45MPH), it doesn't have the power at all. Perhaps streamlining the connection might make enough of a difference to allow 5th-gear cruising, I don't know.....but I can guess I would find myself shifting into 4th every time a mouse in front of me farts.
Would I be damaging anything by spending long periods of time in 4th gear? I'm not worried about the engine RPMs, the car itself is geared so tall that 4th gear has me sitting very comfortably at 3,000 RPM at my preferred towing speed of 60MPH, which is where my old Subaru Outback ran in 5th; I'm more worried about longevity of the transmission (specifically 4th gear) by spending so much time in a gear that perhaps wasn't designed for highway cruising.
I would eventually like to set this combo up as a long-distance fuel-efficient traveling/vacation unit, with an aerodynamic fairing mounted on the truck lid and strapped to the bicycle rack/fender openings/rear door openings, plus a receiver-mounted boattail cargo box for the trailer that would also swing out of the way and serve as a kitchen counter since the kitchen is in the back and with the wheels so far rearward, the trailer is nose-heavy as it is. The car doesn't feel the weight (750 lbs) nearly as much as it does the wind drag on the highway.
