And sorry Marwayne and offgrid, you didn't pull a fast one on my cousin Murphy.  You only addressed the first two elements of the law, which is incomplete.  What can go wrong, will go wrong at the worst possible moment.  Each of you lacked the element of "worst possible moment."  Thus Murphy is still waiting patiently for the proper combination of the three elements of the law.  And he will strike with out warning.  You probably had plenty of gas stations along the way, plenty of cash in your pockets or a hefty credit card limit, and/or agreeable weather making it a lousy time to impose the effects of the first two elements of the law.

And just to cement the notion that we are back on topic, I'll include here an excellent online tool for calculating fuel economy where you can load in your numbers and see the impact of aero drag. I put in .01 for rolling resistance, 8600 lbs wt, left the drag coefficient at 0.34, and plugged in 47 ft2 for frontal area. Left all the other inputs as is.

Comes out very close to my actual numbers. You can see to what extent aero drag on the trailer controls fuel economy. All you have to do to get 18 mpg is drive 50 mph (or have about a 20-25 mph tailwind).  

https://ecomodder.com/forum/tool-aero-rolling-resistance.php

If you're on the southeast coast in the summer, you could just wait for a tropical storm to make your move, depending on where the eye was you could go in any direction, or even go around in a circle if you wanted to. Probably could go pretty fast too, for a little while...

I believe we just participated in another round of the dreaded thread drift...

Well, there wouldn't be much surface area on the sides of the pod, it'd be awfully cramped inside the air foil, and you'd have a hard time making shape switch from port to starboard tack with a rigid foil system..

When we drove from New Jersey to Dayton, NV, to look for a house to buy for retirement, I managed to hit headwinds in both directions.  It's just a question of planning.  The prevailing winds are often redirected by low and high pressure troughs and ridges.  If you want to go to the north west, just wait for a storm and catch the SE wind; a piece of cake.  If you want to cross the country, just follow the opposite of the winds we had when we drove to/from Nevada the first time.  On the east coast, it's common for an onshore wind in the morning and an offshore wind in the afternoon, thus go to the beach in the afternoon, then a couple days later, leave for home in the morning.  

  

 I really like that,that way you would never late

But these are all for empty trucks not towing anything. it looks like we all agree that there wouldn't really be any difference to speak of towing with any of these choices. 

If you have properly inflated road tires and add in wind speed and vehicle speed to your mix of variables that you're keeping the same, I agree. Tow with whatever you want, as long as its "modern", ie, has a multispeed auto tranny, efi and good electronic engine controls, and it will probably show pretty much the same fuel economy on the same piece of highway going the same direction. 

One thing that might help a little is to add one of the roof mounted spoilers to your TV to try to improve the drag coefficient. I'm thinking of trying that just of fun. I have access to the local airstrip here to run coast down tests to see if I can measure the effect, but I'd have to wait for a calm wind day, which are few and far between here on the OBX. 

But yes, it takes "X" horsepower to do "Y" work.. and it takes "X" amount of fuel and "Y" amount of air, to make that horsepower number.

So regardless of engine size, everybody gets the same mileage towing, +/- a few percentage points, when towing.

I was unpleasantly surprised the first time I took my rig to the WV mountains, from sea level to 4000 ft or so. I expected bad fuel economy once I hit the mountains. Instead of my usual 14 mpg plus I got 9.5. Bummer, but I'm getting it back coming down, right?

Nope, I got 11 mpg. Why? Because I have to make 7 2000 plus foot climbs and descents to get to the final climb up to 4000. That's the Appalachians. Then, engine and wheel braking all the way down each ridge heating up the air for next to no benefit. On the way back I have one less climb so its a little better, not much. 

All this is why comparing fuel economy towing is so confusing. Unless you're comparing running a dead flat highway at the same speed in a zero wind condition, there are way too many variables making the numbers bounce around. Even a 5 mph headwind will drop my fuel economy from 14 to 12.5 or so mpg. 

To bring this thread back to rjmarlatt's original question. You probably don't want to get a new tow vehicle expecting some great improvement in fuel economy towing, you're likely to be disappointed. Almost certainly you'll never recover your investment with fuel savings.  Most of the engineering that makes newer vehicles more efficient is washed out when you're dragging around a big heavy box. 

My recommendation is to get a new tow vehicle because that's what you want to drive, make sure it meets your towing and other requirements, and enjoy. If your towing fuel economy improves along the way, that's gravy. And one requirement needs to be a big fuel tank, I'd say about 20 gal is the minimum. Less than that and you'll be looking for a gas station every couple of hours.

I won't go into details, but most legs, no stops. None of it was fun. My family is used to my military ingrained need to get there, get it done, and get back. My baby brother is also prior service, so he was no problem. Mom groaned a little first time up.. but by the last trip down after the funeral, she was "get home, now. not a peep out of her"..

Holy rest stop, furpod! How often did you have to pull over or, did you just dehydrate everyone before?! Are we there yet?! 

fred

Let's assume the same trailer and tow vehicle, the only difference is the engine. HP requirement is aero drag plus rolling resistance. So the aero drag will be identical and the rolling resistance will be nearly so, insignificantly more for the V8's slightly higher weight. And that will be true whether towing or not towing.