When towing, is it better to run the gearbox in fourth or fifth gear?

Conventional wisdom is that an over-drive gearbox (such as your five-speed) shouldn’t tow anything heavy in top gear. The loads placed on the transmission via the over-driven fifth ratio can cause internal transmission damage.

Also, by leaving the engine in fourth gear (locking out over-drive in the automatic version) you stand to have the engine operating squarely in its comfort and torque zone, rather than lugging along in a too-tall ratio.

So that’s the nutshell answer: Save fifth gear for downhills and tail-winds and keep the load off the transmission.