The mail bag is getting full and it would be the polite thing to do to answer the questions and make room for more. Santa Skip wrote the following after I used the term Tommy Knocker:

“I spent my younger years exploring the ghost towns of CO, to me a tommyknocker was a little elf who lived down in the mine, if he liked you he would show you where the gold, (was) if he didn’t he would cause all kind of problems.”

One of the neat things about traveling is that you get to learn that an expression that you grew up with have different meanings regionally. But this is a new meaning of Tommy Knocker to me.

Chuck wrote the following after my comments on driving at 55 miles per hour:

“I’m posting this as a reality check. I’m getting better MPG at 68-72 mph (2200 rpm) than at 55 mph. I drive a F250 with a V10. With or without the 5th wheel trailer I get better mileage at a higher speed and higher rpm. At my last fill up I got 10.8 mpg. I traveled 107 miles with the 5th wheel trailer and 190 miles without the trailer. In the past if I drove at 55 I would get around 8 to 9 mpg. Has anyone else seen this type of mpg driving faster instead of slower?”

Under real world conditions it is almost impossible to get accurate fuel economy numbers. Back in the dark ages, about 1980 something, the EPA was in the process of certifying fuel savings devices. They invited a group of automotive writers to a test drive to provide the EPA with numbers of fuel economy improvement of the new Volkswagen E Shift light system. It was a computer controlled device that would turn a light on on the dash that would signal the driver when to up shift for best economy. The route was from the tip of Long Island to Newport, Rhode Island. Lots of rolling hills.

VW teamed up the drivers and it happened that Dan Holt, then a writer/editor with the Society of Automotive Engineers publication, and I drew the duty as partners. Now not to say that we were heavy, but between the two of us we exceeded the load carrying capacity of the car, not counting our luggage or our camera gear. Now as every one knows writers have imagination. Dan and I being both engineers decided that we would show some real fuel economy, and here is how we did it.

The first thing that we did was to eliminate rolling resistance. We stopped and pumped the tires up to 80 PSI. The car rode like it had Freddy Flintstone Hard Rock tires. Then we applied some simple junior high school physics. Gravity. We figured with our combined weight, the weight of the car, and gravity, that we could use the engine on down hill runs to get the speed up, and then coast up the next hill. Foot to the floor in high gear going down hill, put the tranny in neutral and shut the engine off for the uphill coast. In several instances on the run we were able to leave the engine off for several hills as well as seeing the tach hit red line in high gear.

VW controlled the record keeping of fuel in each car with a burette attached to the front bumper and at each fuel stop they measured fuel used and filled the burette to the full line and we ventured out again. Well Dan and I placed second in this 50 car run with around 100 miles per gallon. A couple of guys from one of the magazines out did us by a couple of miles per gallon. I think they ran a bit more air in their tires but they used the same techniques. We did show VW that we could get good mileage but it did little for the certification of the Up Shift Light. And we broke speed laws and really had a ball.

The moral of the story, which is a true story all the way, is that determining a difference in fuel economy means measuring apples against apples under like conditions with only one variable at a time. Driving cost over an extended period of time is a valid number.

Leave a Reply


  1. Dave

    I have a 2007 5.4 litre F150 crewcab which I pull our ultralite 18.5 ft. trailer (3,700lbs)

    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?” 😉

  2. Jim Norman

    Fuel economy is something that most people fail to calculate correctly. Examples of the wrong way include: Running a vehicle out and then adding one gallon and measuring how far you go, (Probably the absolute worst way), calculating mileage on one tank, one trip, much better, but certainly not an average nor a realistic view of what you will get under varying conditions.

    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.


  3. bob

    I uesed to do MPG rallys in Sacramento ca. years ago with a VW bug. We also did all the things you did and more Graphite in the wheel bearings, and the big one was DRY ICE on the gas tank all night befor the rally, It would shrink the full gas tank by almost 2 gallons, then fill up at the start and as the gas got hot, it would expand and at the end of 2-3 hours of driving it was still FULL and I got way over 100 MPG at the end, NOT REAL WORLD DRIVING but it was just for the fun to see if you could get it. bob

  4. TXBrad

    I agree 100% w/ David Mac Donald in that each engine has a “sweet spot” ( optimum rpm) This can be impacted by engine oil used, miles on engine, & many other factors. Combined w/ engine is gears rear end & transmission, tireswheel size etc. Key is to figure the best speedrpm for YOUR engine in your vech. .Drive similar trips @ 55 [ check rpm & mpg }] then run @ 60, then 65, 70
    hopefully, hills, weather, wind, vech. weight, etc. about the same. Then drive by that sweet rpm. If it drops off ” up abit or down abit.
    Gasoline in central Texas has “up to 10%” ethanol. I agree w/ 2-3 less mpg in my pickup. My son’s 2008 Full size pick-up using the E-85 fuel gets 11 mpg.
    My 31 foot Motorhome gets 10 mpg on 10%. Going cross country w. & w.o. added ethanol got 12 -13 mpg w/ some use of generator ( lunch/rest stops).txBrad

  5. Paul Kunkel

    “So, what mileage you get depends on where you fill up when it comes to gasoline.”

    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.

  6. David MacDonald

    I would like to respond to both David Campbell and Chuck on the subject of mileage. I have a diesel 6.0L F350 SRW with Crew Cab and long bed 2007 Ford. At the scales it weighs 8350 on average.

    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.

  7. John Holveck

    I have a 2008 F350 DRW King Ranch. My third diesel pickup, 2nd F350. First diesel pickup was an F250. My trailer weighs in the neighborhood of 14000 ready for a trip.
    About 22500 total gross between truck and trailer. I bought the 2008 because Ford had uped the combined gross wt. and now I’m not running over their (Fords) gross wt. limit. On all three trucks I get better mileage if I keep my speed above 60-65 MPH. The 2008 doesn’t seem to pull down at slower speeds like the other two did (I think it probably has to do with the improved turbo) but it also gets the worst mileage of the three both solo and towing. However saying that it also pulls better that the previous two. The 2000 model with the 7.3 engine actually pulled better than the 2004 model although it needed a gear between 3rd and fourth. they solved the problem with the five speed transmission.

  8. Ron

    I must admit doing my own Unscientific test I also have concluded my 38 foot 8.1 NEWMAR Mountain Aire gets better Mileage at an increased speed no matter the terrain. The funniest thing about this is that when I was in the Transportation Department of one of the fortune 500 Distribution Company I did seminars on the increase fuel mileage as a result of decreased speeds. Glad I am retired now. Just my two cents worth.

  9. David Campbell

    I would add to the first post, running a Class C motor home, Ford V10 at around 2200 RPM cruising. At 3000 to 3100 or so, when necessary climbing hills and letting the speed drop off with a more or less constant RPM, letting the transmission shift when it wanted. The average MPG on the flat ground will run up to 9.0 and then in rolling terrain like western South Dakota it will drop to just under 8. Then an overall average, Seattle to Fairbanks and return was 7.5.

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

  10. I would like to respond to Chuck here, if I may…

    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.

  11. Chuck

    Brad, I have been getting the results of better mpg at a higher speed and higher rpm since last summer. However I decided to carefully document a trip from Portland ME to my home outside of Boston and that was the detail I provide in my post. I’ve been keeping mpg records since I purchased the truck new, however until last year I rarely drove over 55 or 60 with or without the trailer. I first noticed the high mpg after rushing home when my daughter had a baby. The trip home was 304 miles and I got 14.0 mpg (no trailer) That is the highest mpg I had ever seen with the truck. I then started to pay attention to the mpg difference at higher speed and it seems counter intuitive. One theory a friend of mine had was that the V10 may need to be at 2200 rpm to run most efficent. I don’t know the reason but I’m sure curious because every time I drive between 68-72 mph I get better mileage than at 55 mph. I agree with you that it is near impossible to get totally accurate mpg in the real world but the trend of higher mpg is reproduceable every time. The numbers change but they are always higher!
    P.S. I know what a tommyknocker is I drove 18 wheelers for 12 years when I was younger and you are correct a low tire makes a different sound than one that has the correct tire pressure.

  12. David Campbell

    On the topic of measuring fuel economy, you are right in using an overall fuel usage over a measured distance. But, has anyone checked the accuracy of the onboard computers in today’s vehicles? The figures that I get using a ScanGauge give me numbers that show that running at 58 to 62 mph, depending on terrain, gives a better mpg than at 55. That is in addition to doing similar techniques in rolling terrain as the above author did, ie. whoosh down the hills and crawl up the other.