As promised, here are the details of my recent trip to Morvich and Culloden with my ML320 CDI towing a Bailey Oklahoma.
Total mileage was 704, a good bit higher than estimated before the trip. Overall MPG was 22.98mpg and it was broken down as follows:- Aberdeen to Morvich (against a very strong headwind - the only respite was travelling down Loch Ness from Inverness to Invermoriston where the land mass sheltered me from the westerly wind), 174 miles at 17.3mpg, avge speed 40mph. Morvich to Culloden was 67 miles at 19.3mpg, avge speed 40mph and Culloden back to Aberdeen was 96 miles at 22.4mpg, avge speed 44mph. Travelling solo, I was often getting 28 - 30 mpg.
I tend to sit at 60mph when towing but, in addition to passing through towns, there were delays at four sets of roadworks which quickly pulled the avge speed down.
While the fuel consumption when towing was not as good as anticipated, virtually all the mileage was on ordinary single carriageway roads. I don't think I will be towing on dual carriageways or motorways in the near future so I'll need to wait and see what that gives. At the end of the day, I am not too worried about fuel consumption as there are other, greater costs to be concerned about. The consumption "is what it is" and I'll live with it. Incidentally, I didn't require to refuel until I reached Inverness enroute to Culloden, 452 miles after leaving Aberdeen. The car took 96.12 litres of diesel when the tank is supposed to hold 95 litres! The computer display showed zero miles of fuel remaining. I'll try not to cut it so fine in future!
The ML is a nice car to drive and it tows the Oklahoma effortlessly. The 7-sp auto box is really smooth and down changes occur very quickly, and smoothly, when the accelerator is floored.