I check all the tires, load everything correctly, and get pretty horrid mileage, considering I get fairly decent mileage without the trailer. Oddly enough, without out the trailer,I get better mileage (23-25 mpg) at slower speeds (70-80kph) than at posted highway speeds (19-20 mpg@100-110kph (70 mph) , and it drops down anywhere from 6 to 12 mpg depending on hills/wind etc. Although we love travelling, this is going to curtail our long distance plans for sure. We were relieved that we did not buy a trailer anywhere near the allowable rating LOL! At least it gets better numbers than my Jeep Cherokee. I suppose the comments will be in the order of..”suck it up Canuk-boy…what were you expecting?”

I have done the following steps to calculate my mileage.

Set up a spreadsheet, enter starting mileage and ending mileage for every tank, enter gallons to the hundredth and also cost to the penny. One calculation is the ‘current’ MPG and $PG. I also calculate a lifetime MPG and $PG and recently added a 3 fill average. Also added in is a percentage change where 100% plus is an increase in MPG over the previous calculation and under 100% is a decrease.

I have a 2007 Itasca 32K with a Ford V10. I have a current average of 6.8 MPG, although my last two tanks are 6.9+ taken as an individual reading. This is a very new unit, only on its second oil change and with about 5500 miles. I think I may be seeing a tread to higher mileage as the engine ‘breaks in’.

The last couple trips have been while towing a Jeep Liberty and driving thru the Blue Ridge and Shenandoah Valley.

I am looking at possibly changing to synthetic oil in the future. Also, I have noticed that dropping out of cruise control on hills allows me to keep the revs between 2400 and 3500 although sometimes I find I have lost some speed. If I leave the cruise on, I seem to drop two gears and hit well over 4k. Other item is I don’t engage the ‘Tow-Haul’ except going down hill where the engine braking helps.

Jim

I agree. I used to fill up at the Indian casino stations because the gas was considerably less expensive. I got worse milage but it was cost effective until they raised their prices to 7c under the mainline stations.

I just returned home from a trip across the U.S. and back for a total of 10116 miles and I got 11.73 mpg for the total average on this trip. I checked every fill up and I never got below 11 mpg and sometimes it was 12 and on a rare occasion I got 13. Only once did I get 14. All of this with pulling a 30′ 7″ 5th wheel Alpenlite trailer that at the scale weighed in at 11,500 lbs. Without a trailer on get between 18 and 21 mpg on the highway with 24 mpg being my best. And all of the above is before the diesel is even broke in as the odometer is currently reading a little over 16,000.

After saying all of this I would like to identify that every engine or electric motor has what I call a “sweet spot RPM”. This means at a certain RPM a motor or engine will be at its peak performance and therefore using up energy source most efficiently at that RPM. I asked quite a few Ford dealers what the “sweet spot” was for my diesel and nobody knew until one day at a dealer I asked the shop foreman that question. His response was what is the normal highway speed limit in this country and I responded with 65 mph. His response was, what ever the RPM your engine is showing at 65 mph that is the “sweet spot” that has been designed into all automotive engines in this country. I’ve tried doing a steady 60 mph pulling the trailer then running on average 63 while varying between 60 and 65 and for the former I would get 11 mpg but at the latter speed I would edge toward 12 mpg.

There is currently a senator in the U.S. Senate that wants to change the national speed limit again but has asked the DOE to identify what the MPH speed is best. Currently the DOE says 60 mph is the best speed but like the average 12,000 miles per year we all put on our cars this 60 mph is most likely very out-of-touch with the manufacturers. One other thing I did was not see how fast I could run up every hill I came to but instead held the RPM about 400 to 700 above the 65 mph RPM setting.

There is one other fly in the ointment in this whole issue fuel mileage and that is “watered down” gasoline. I currently have no information whether this is ever the case with diesel so these comments are for gasoline only. I lived in the Minneapolis area for 22 years and without fail I would get only 32 to 35 mpg on a tank of gas from a station in the cities. Before Ethanol it was 2 to 3 mpg more. But, if I filled up at a small country town at least 50 miles outside of the Minneapolis metropolitan area I would get easily 37 minimum and often 39 maximum but occasionally it would go to 41 mpg. My theory was the gasoline out in the farm communities was less watered down just because the farmer needs all the help he can get and why penalize him. The farmer also probably kept very close track of mpg usage. When I would travel I would see this same thing happen anywhere I went across the U.S. So, what mileage you get depends on where you fill up when it comes to gasoline.

Variations due to terrain are really the most important determining factor in the average mileage along with the way it is driven.

A lot of factors are involved with determining the best speed for milage on a given vehicle. Almost all of it is centered around how much load is on the engine at a given speed.
I will say that with a pickup truck, the concept of dropping the tailgate is pure bunk; it has been practically and clearly debunked on a show called “Mythbusters” on television. They first tried to use a wind tunnel to measure the drag created by a truck in small scale; but their attempts met with failure due to too little measurable difference. However, when using a ‘water tunnel,’ the current flow over the truck and bed was very visible. The open-tailgate model had significant drag that broke up the airstream over the vehicle and eddied for a long distance behind the truck. By closing the tailgate, it created a specific wind-eddy in the bed of the truck that actually helped the airflow over the bed and cut the visible disruption very close to the vehicle by a factor of 2x to 3x closer than with the tailgate down!

But going back to my original point, engines start losing load at different points. Most 4-cylinder cars are actually designed to run between 2500-3000 rpm at cruise; at the bottom end of their power curve but not below, where it would be loading it just trying to maintain speed.
Of course, the more aerodynamic the car, the lower this number needs to be. In a 1996 Camaro, for instance, the body is so slippery that the 3.8L V-6 could run all day at 1500rpm and not be loaded. Your truck, however, is essentially a brick shoving end-first into the air, creating a huge pressure wave. Even though your V-10 produces a lot of torque, your power band is still better than 2000 rpm, and with some engines where the cylinders are smaller than an equvalent-sized V-8, that means the power band is even higher. Oh, yes, you get great torque; but you also are turning faster to get it.

I have a similar issue with my Jeep Wrangler. Nearly everyone I talk to says they are lucky to get 18mpg on the highway; most of them driving 65-70 mph on the Interstate. I, on the other hand, have broken 25mpg by pulling myself down to 62 mph or about 2300 rpm. An additional point I may make here is that most of the folks I’ve talked to about their Wranglers rarely shift into 6th gear.

This brings about another point; there’s such a thing as taking too long to accelerate. The more time you spend getting up to speed, the more fuel you are burning. This doesn’t mean you should ‘jackrabbit,’ but smooth, strong acceleration to highway speed has you at cruise and at best economy sooner than taking 2x – 3x the distance.

I don’t know if you’ve ever watched “Top Gear,” a BBC auto-review and performance program (NBC is creating an American version right now.) but Jeremy Clarkson, one of the hosts, clearly demonstrated that, “It’s not the car you drive that counts, but how you drive the car.” He took a BMW M3 and followed a Toyota Prius around their “Top Gear Test Track” at the Prius’ fastest speed. They precisely measured the amount of fuel burned in 10 laps of the track and calculated the MPG of both vehicles. The Prius got something like 16.7mpg in the test. The BMW got 19.6. Yes, the high-performance sport sedan got better gas mileage than a Prius at the exact same speeds.

No, I’m not knocking the Prius or any other hybrid. I’m only pointing out that it’s how you drive that car that will make the difference. Every car is different, and to expect that a single, set speed limit will ’solve all the world’s gas problems’ is taking an extremely unrealistic view of the issue. Driving carefully and within your car’s capability is far more effective towards economy than hot-rodding or over-compensating for increased prices. My guess is that those who are driving too-slowly are actually seeing worse gas milage than they were before they changed how they drove.