pulling with a 2011 F250 6.7 pulled a 13000 lb from fl to el paso got approx 12 mpg. Traded down to a 28 ft 7600 lb, same conditions,same speed same mileage. I would think that with pulling 1/2 the weight I would have seen an increase in mpg. Comments from the experts please.
Maybe a little more power used to get the old one rolling, but once you are cruising it's all wind resistance. If your smaller fiver has the same frontal area as the old one, then I doubt you will use less fuel to pull it.
My current fiver is 2' lower profile (and 5000# lighter) than my old one and I did see about 1.5mpg improvement.
1999 Sunnybrook 27RKFS Fiver.
2005 GMC Sierra 2500HD CC/SB/DA 2WD, LBZ air cleaner, 52 gal Titan tank, Bilsteins, Line-X, Westin steps, Prodigy, Retrax cover, 16K Superglide, 5th-Airborne pin-box, Multi-vex mirrors, TST TPMS.
Why on earth would you think that changing the weight of the RV would change your towing mileage? The factors that affect how much power you need to tow aren't hardly at all weight. Weight affects acceleration more than anything.
1. wind resistance
2. shock absorption
3. average road slope
4. tire rolling resistance
5. bearing resistance
1 - wind resistance is related to the cube of your speed. This means the faster you go, the more wind resistance there is. The number one factor in *all* vehicle efficiency concerns is wind resistance, which is directly related to frontal area. The frontal area of a fifth wheel is 12 feet tall by 8.5 feet wide. BIG.
2 - Shocks. This is more power than you might think - EVERY bump you hit, those shocks are absorbing energy - in proportion to your weight. And that energy comes from your momentum.
3 - The most obsessed about factor that doesn't really count for much change in fuel mileage. Going uphill, you use more fuel, you're running at max power. Going downhill, you don't use noticeable fuel at all (in a diesel). Mountains have both UPhill AND DOWNhill. Wind resistance is still a significant factor if you're doing it at more than 55mph or so. Certainly between florida and texas, any hill you went up, you came down again.
4 - Minor. Small enough that you could probably overcome the tire rolling resistance of your entire truck *AND* trailer by leaning against the back of it. It wouldn't move far before it stopped, but it would move.
5 - even more minor. Small enough that a 10 year old boy could make the whole rig move by leaning on it, if that was the only resistance.
And how do these change when you change the WEIGHT of your RV?
- the biggest factor wind resistance stays the same
- Your shock absorption rate increases - but the factor is tiny compared to the change in weight
- your truck STILL runs at max power on the mountains, just goes a bit slower uphill, which cuts wind resistance, attenuating the mileage drain effect.
- your tire rolling resistance increases very slightly
- your bearing resistances increases infinitesimally
Add up the difference in shock, mountain climbing minus wind speed change, tire rolling, and bearing resistance, you'll find it costs you a remarkably small amount of fuel. If you want to save fuel, get one of those travel trailers where upper portion of the outer walls sink down around the lower portion shorten to their height to about 2/3rds. THAT will save fuel.
I just went from a 30" Cougar TT to a 35"8" Montana 5vr, pulling with a 2010 Chevy 2500 duramax. I got the best mileage I have ever had towing from Calif through Oregon and Washington with the 5vr - even though about 4000lbs heavier. Not sure if its more aerodynamic or if because weight is centered on the truck axle. But - I'm surprised and happy.
I'm beting that the conditions really weren't the same. It doesn't take much in the way of wind, or hills to really change the mileage. I wouldn't give up on the idea that your mileage will be better in the long run. I don't buy the idea that mileage is going to be the same regardless of trailer weight as it takes less energy to pull less weight. It just may be that the difference isn't always dramatic enough to notice or that it is inconsistent.