As you know from Is there a green way to tow?, there are a number of things that impact your gas mileage when towing. In addition to the weight of your load and how bulky it is, factors like how fast you drive and whether you're using a gasoline or diesel engine will play a role.
While diesel fuel is usually more expensive than gasoline, diesel engines can get 12 to 15 percent more power out of a gallon of fuel, so depending on how much you tow, the trade-off could be worth it. In addition, diesels tend to have more pulling power than gasoline engines, enabling them to tow heavier loads without working as hard [source: Murphy].
And while conventional wisdom says the bigger the engine, the worse the fuel economy, when it comes to towing, bigger may actually be better. Larger, more powerful engines don't see as much of a drop in gas mileage when towing as smaller ones do. They simply don't have to work as hard because they're designed for that extra load. So even though large vehicles never get excellent mileage, when compared to small or medium-sized vehicles that may only get 60 percent of their rated fuel economy when towing, these larger engines may actually come out on top [source: Murphy].
It also matters where you'll be driving. If you encounter a lot of stop-and-go traffic, the drain on your mileage will be greater than if you're traveling on long stretches of highway. Again, this is Newton's doing: According to the law of inertia, objects in motion tend to stay in motion and objects at rest want to stay at rest. In towing terms, a lot of stopping and starting puts more of a drain on the engine's energy use because you have to repeatedly get that heavy mass rolling. But once it's going, such as on the highway, momentum helps to keep it going so the engine doesn't have to keep exerting as much energy as it did initially.
As you can see, because of all the variables involved, it's hard to say with certainty how much of a drop you'll actually see in your mileage when you tow. Experienced towers cite a decrease from 5 to 10 mpg on average and note that they definitely see more of a difference when traveling at higher speeds [source: Tundra Solutions].
At today's gas prices, even 5 mpg is a lot of moolah, but as long as you're not high-tailing it down the interstate at speeds greater than 80 mph (129 kph) with a double-wide trailer in tow, you should be able to manage at least a trip or two. For more on towing and gas mileage, haul yourself to the next page.