Is there a MPG rule of thumb when towing?

Interesting discussion, I think the rule of thumb approach to economy is more important than anything else, specifically when towing.

Whilst many people slip into a style of driving that suits them when solo, and each individual gets a different mpg as a result, even using exactly the same car, the main reason for high mpg when the driver is taken out of the picture, is tyre pressures, road rolling and wind resistance, and the load.

Best address those before addressing your driving technique;

For example, how often does the average driver check their tyre pressures?  The recommended minimum is weekly, for emergency vehicles it's the start of every shift, but how often do you check them?  It's even more important your tyre pressures are correct on your outfit before it moves down the road, reducing rolling road resistance.

Secondly, rolling road resistance vs wind resistance; there's numerous studies that show that these both rise significantly at 56 mph, in calm conditions, so the general advice is that driving either solo or an outfit beyond 56 mph severely impacts fuel economy.

Thirdly, the heavier the load, the more energy is required to move it and stop it, so if you're more concerned about economy than comfort, then the lighter your outfit should be.

So tyre pressures, 56mph top speed, and no excess load.

Then you could have your engine remapped for economy (solo) or towing (outfit), incidentally with increase in the amount of torque at low revs (towing remap), you can also short shift a manual gearbox when driving solo to improve overall economy.

Finally, it's you, the driver that needs to change the way you drive, generally anyone that has undertaken LGV, Advanced Driving, or Emergency Driving in the past, will have vehicle sympathy, better hazard anticipation, better throttle/brake control, and appropriate gear selection that improves economy overall.

So for a car driver, undertaking any Advanced Driving Course and one of the Caravan Club Caravan or Motorhome Courses is a very good idea.  You'll be safer overall, and your economy should improve, and your car will last longer before needing brakes and tyres.

I get up to 65 mpg from a 2.0 Diesel Toyota Avensis Estate with a towing remap when driving solo, short shifting the 6 speed manual gearbox whenever the revs get to 1950 rpm.

I've only ever towed my caravan once when I bought it and moved it to my secure storage, but I will have to change my driving style even after that one trip, it needs more rpm between gear changes, the outfit slows down more significantly due to wind resistance requiring less braking pressure and more anticipation of the road ahead, and will require using 5th gear on single carriageway roads, and 56mph in 6th gear on Motorways and dual carriageways.

I think if economy is more important that your itinerary, then using a weather app to avoid headwinds, and studying your touring route to avoid steep inclines would be more of a priority.

So I'll be using the above as a 'Rule of Thumb', and my wife will determine our itinerary, and we'll suck it and see.