Weight vs frontal area

wintersun wrote:
At 60 MPH the air resistance is 4 times what it is at 60 MPH.........


..... and if you are going 70 MPH and have a 20 MPH headwind your gas mileage is going to be very poor. Drag and turbulence also increase the power needed to move any object and it would be difficult to come up with a better shape to maximize both turbulence and drag than a pickup pulling a square trailer. For a good shape look at a Prius. .

I think you have a typo in that first part friend, I think the second '60' should be '30'.

Often the gear ratios for a truck are engineered for maximum EPA stats and not for towing. A 4.10 rear end will enable a gas engine to provide power in its sweet spot or power band at towing speeds. Taller gearing is fine when driving at 70 MPH with nothing in tow but keeps the engine operating at suboptimal RPM's when towing at slower speeds.

Weight take energy (gas) to accellerate and to move to a higher elevation. Rolling hills are the worst as the engine speed is constantly changing and you cannot take advantage of the downgrades to help you up the next hill.

At speeds over 50 MPH most of the work the engine is doing is to overcome air resistance which increases with the square of the velocity of an object. At 60 MPH the air resistance is 4 times what it is at 60 MPH and if you are going 70 MPH and have a 20 MPH headwind your gas mileage is going to be very poor. Drag and turbulence also increase the power needed to move any object and it would be difficult to come up with a better shape to maximize both turbulence and drag than a pickup pulling a square trailer. For a good shape look at a Prius.

The only "RV" that gets decent fuel mileage is the Class B motorhome with a diesel engine which is capable of averaging 25-28 MPG. With anything else it is best not to think about "economy" at all. More important is range on a tankful of fuel as that can have an impact on your trip planning.

JaxDad wrote:
In other words the drag at 60 mph is way more than double what it would be at 30 mph.

You think about 4 times, maybe?

Back in 1962 there was a big storm in the Black Hills with a lot of flooding. We were coming East at that time with a 21' Layton TT that weighed about 4500# and towing it with a Lincoln. The combo had given over 10mpg on the trip West but that day I could not get over 50mph and got 5 mpg. The wind from the front did that much to the speed and mileage.

It looks like a couple of points got missed in this thread so far.

First, don't underestimate the drag of tires on the road, a tandem axle trailer that weighs the same as a single axle trailer will have more rolling resistance (4 tires versus 2) which equates to more gas.

Tires themselves make a difference too, taller tires means larger circumference and less resistance to rolling than smaller tires. The inflation of the tires also makes a big difference.

As far as frontal area goes, there's lots of variables, sloped back versus verticle, wedge shaped versus flat, tow vehicle size, height and width.

Also wind resistance is an exponential loss factor, the faster you go, the more drag it creates. In other words the drag at 60 mph is way more than double what it would be at 30 mph.

Allworth wrote:
Slow Down.

I have a one ton diesel that would pull a house off of the foundation if I had enough chain, but I get the best fuel economy at about 55-57 mph. Above 55 I give up about 1.5 mpg for each 5 miles per hour increase. Not usually worth it.

+1

Slow Down.

I have a one ton diesel that would pull a house off of the foundation if I had enough chain, but I get the best fuel economy at about 55-57 mph. Above 55 I give up about 1.5 mpg for each 5 miles per hour increase. Not usually worth it.

Slow down. Most rver's don't race up hills. From reading different forums, it seems that most have found their best mileage at 55-62 mph on flat roads and go at whatever speed up the hills doesn't stress the engine.

I vote that frontal area is the biggie!

It's no big deal to get the camper up to 45 miles / hour. Keeping at 70 takes work and a headwind or tail wind is BIG on mileage and how hard the engine works!

I have towed heavier stuff- and going down the highway was much less 'work'.

It'd be wrong to say that weight doesn't matter- but wind resistance from a crummily streamlined front end and a total flat rear end is killer to towing.

Gas engines towing high walled RVs means 8-10mpg. Diesels 12-14mpg. As mentioend, weight is a factor when accelerating, but most RVers travel highways at 60-65mph where aerodynamic drag is the fuel cost. 4000 pounds or 10000 pounds, same FE for the same truck, assuming it has the capability.

It is wise to consider the TV and RV combo at the same time. IMHO, a very capable half ton should be the minimum TV. A couple get about 17mpg mixed, about the same as my last minivan and the same as most mid sizers. They have the capability to comfortably handle the size and weight of RVs. If youare considering 5000 pounds loaded RV, 4k dry, then check out the 2014 Jeep Grand Cherokee diesel and VW Touareg TDI. They should both drive and tow very well while offering improved fuel economy (15mpg towing, 25-30 non towing).