Re: how to increase miliage when towing
There are so many things to consider in this discussion and so hard to respond to each post and all of the good and, maybe not so good, info. I have been in the heavy-duty truck engine business my entire career. Fuel economy improvements are what I have focused on for the last 15 years. It is important to note that
economy is not always about efficiency. Ultimately you want this stuff to save you money, and Scott's comments regarding payback are extremely important. I happen to like efficiency stuff just because, but you need to decide if this is about money or not. If it is about money, then very few of the items being discussed will pencil out. Even some of the less costly ideas. Of course I am not talking about air pressure, but even a $700 custom tow cover will be hard to pay back for most of us. If you need a new cover anyway, then that's different . . .
I have told a story here before about "Magic Gas Pills". Of course they were bogus, and are basically moth balls. But people I respect were reporting better MPG, and that absolutely convinced me that anybody can be a victim of the Placebo Effect even with something that should be objective like fuel mileage. People want these things to work, and somehow that manipulates their brains and their driving habits to a point that they report even 30% improvements. Trust me, vehicle OEMs will spend a TON of money for 3%, and these people are reporting 30% by throwing moth balls in their tanks :facepalm: You don't think OEMs know how to spec an air cleaner? CAFE is a flippin' HUGE deal.
There are also some weird comments regarding required horsepower for given road loads, and I suspect the posts are just incomplete. You really can't make blanket claims about thermal efficiency and then discuss different displacements and technologies, and draw any conclusions from that. The fact is that every single engine speed point (RPM) and every load point at that RPM have different thermal efficiencies. This is especially true for an Otto cycle engine as I kind of mentioned above. Diesels maintain their thermal efficiency better across the entire load and speed map. And this BTW, is kinda why these rear end ratio discussions are actually bass ackwards. Yes, some people report better fuel by raising the numeric ratio to reduce lugging (see Placebo comment), but the FACT is that a lugged engine (Otto for sure, and diesel sort of too) is better on fuel. So the thought that you can improve cruise efficiency by changing the rear end is pretty much false. You might as well tow in third gear if that's what you want. Why isn't that better on fuel?

Same thing with a vacuum gauge. If high vacuum is the goal for good fuel efficiency, then just drive around in 1st gear. higher vacuum than the same road speed in 2nd, and higher still than 3rd etc. So why not?