We are considering the purchase of EITHER a GL class mercedes with a 6 cyl Diesel, OR an ML Class Mercedes (GLEClass as of 2016) with a smaller 4 cyl Diesel. The major consideration is that we pull a small camper (2500-3000 lbs) long distances on a regular basis, and so we are interested in gas mileage. The one gets 26 mpg while the smaller diesel gets 29+. Both are rated to pull at least twice the weight of our trailer.
Our experience has been with our 2010 F150 pickup that gets 20mpg on the highway, but with our trailer behind we rarely get more than 12mpg. I've always felt that if we'd bought the larger engine our regular highway mpg may be less, but our mileage with the trailer would be more. My question is... Is this true? If we go with the 6cyl Diesel with a lot more torque and substantially more horsepower, will the differential between the mileage we get on the road and on the road with a trailer be less drastic?
In general, an engine with more horsepower and torque is likely to show less difference under the strain of towing than when not encumbered with additional weight. Diesel engines are especially well known for their durability, and performance when towing. If you know you will be doing a lot of towing, you might be better served with the larger engine.