Originally Posted by cramar
I assume that this is addressed to me. When towing, it was 60 mph or under. When solo it was 70mph on highway. A better comparison would be solo at 60mph. I suspect the 22mpg would bump up a bit. But I did notice that instead of running in 5th while towing, if I let it run in 6th, I get a slight 3% improvement in mileage. But I would rather run with slightly more revs in 5th than in OD, which is used of course during non-towing.
Never used cruise control (CC) with either run. Unless on absolutely dead level ground, I can do better than any cruise control at reducing fuel consumption. That is because CC uses constant speed, which demands noticeably more fuel up a slight grade and reduces fuel going down. I tend to drive with constant throttle (i.e. constant fuel), which means I slow down slightly going up a grade and speed up going down, which gives me added momentum for going up the next hill. Also I can anticipate slowing down and try to coast as much as possible, which uses next to nothing in fuel.
I realize folks "believe" they can beat CC, but at the end of a day, they don't. Among professional drivers it is contraindicated in large studies. Men motivated by significant bonuses. The CC is constant, drivers aren't.
I was beating CC more than forty years ago, and I know how much harder it is today with far more sophisticated drivetrains. But I'll contend the attention we pay to a dead-on speed is offset the wrong way towards our overall best practice.
The exception may be in rolling terrain under light load. And how aggressively CC software is done. And for a limited length of time.
But for both, the longer the drive, or with only slightly reduced attention, the machine will win.
"Repeatable results" is a better way of looking at an overall annual towing average (versus CC or not CC). And engine time versus trip length.
The worthwhile experiment about CC is to set the speed for CC use about 2-mph below the manual input speed for rolling terrain. The engine time will be close to the same for the trip. And the bonus is a more rested driver with more alertness to mirrors and potential problems at a distance. Try it.
Ok, so 35% fuel penalty change is beset by variables. If it turns out you're willing to make the A-B comparison of a similar truck load at same speed and use of CC, this would make for replicable results by other owners of the same truck once exact truck spec, climate and terrain are factored. If the drivers manual says to tow in a particular gear setting, then that applies as well.
RV forums are beset by threads of others wanting to know about a particular trucks fuel cost. But we have "answers" in posts that may as well be random numbers. Speed, load, etc, that are far different, truck spec, climate and terrain that are far different, right down to seemingly minor differences like the use of CC. A range that really isn't realistic for what a truck can do.
Among professional drivers, doing the exact same work in the exact same truck, the gap is just over 30%.
As I'll guess most will report economy over a small range of trips, and from 60-65/mph, the FE may not be so great a discrepancy as 30% between owners of similar rigs.
However, the real numbers are over the largest average number of miles. And over vehicle life. RVers seem to trade vehicles -- trailers or tow vehicles -- pretty rapidly (about six years on each), so results are also skewed towards "new". A higher cost per mile overall no matter the trip fuel economy.
An EB Ford, taking into account the above (lack of reporting protocol) can be made to look better than it is, or so could the EcD Dodge. Or worse.
As the drivers dashboard readout of fuel economy seems fairly accurate today, part of a solo highway trip on CC yet at towing speed (say 75-miles or so) could be offered to "see" what the towing penalty amounts to in an unbiased manner. Truck load is the variable.