Tow Vehicle Fuel Consumption

[Bigfoot Mike]

I know that (some of the rest) of the moderators frown at excessively technical posts but I'm not above tweaking things once and awhile.

I think technical info is a good thing.

[peterh]

The power required to overcome the aerodynamic drag is given by:

P_d = \mathbf{F}_d \cdot \mathbf{v} = {1 \over 2} \rho v^3 A C_d.

Converted to layman's terms

There are amount of power (energy) needed to push your car through the air is

For 60mph:
It takes 27% more energy to push the air aside when you're driving 65 instead of 60
It takes 59% more energy to push the air aside when you're driving 70 instead of 60
It takes 95% more energy to push the air aside when you're driving 75 instead of 60

And 65:
It takes 25% more energy to push the air aside when you're driving 70 instead of 65
It takes 54% more energy to push the air aside when you're driving 75 instead of 65

So the short rule is: At freeway speeds increasing your speed by 15 miles an hour roughly doubles the amount of gasoline required to overcome wind resistance. You can save a whole mess of gasoline by slowing down five or ten miles an hour.

Another factoid: Most cars get their best gas mileage at speeds between 47 and 54 miles per hour.

So, here's the table comparing efficiencies at speeds over 55mph:
It takes 30% more energy to push the air aside when you're driving 60 instead of 55
It takes 65% more energy to push the air aside when you're driving 65 instead of 55
It takes 100% more energy to push the air aside when you're driving 70 instead of 55
It takes 154% more energy to push the air aside when you're driving 75 instead of 55

And, even more impressive, for 50 mph:
It takes 33% more energy to push the air aside when you're driving 55 instead of 50
It takes 73% more energy to push the air aside when you're driving 60 instead of 50
It takes 220% more energy to push the air aside when you're driving 65 instead of 50
It takes 275% more energy to push the air aside when you're driving 70 instead of 50
It takes 336% more energy to push the air aside when you're driving 75 instead of 50

See the [b]BIG jump at 65 mph? We, as a country, could save a whole mess of gasoline and reduce our greenhouse gas emissions by a [b]HUGE amount by setting our metropolitan area (where the highest number of freeway miles are driven) speed limit to 50 or 55 mph and our rural area speed limits to 60 mph.

Science stuff:

Spelled out mathematically, the amount of energy required to push the air aside is:
1/2 the thickness/pressure of the air
multiplied by your car's windface (the total area of your car or trailer facing into the wind)
multiplied by the amount of aerodynamic drag for your car/trailer (more aerodynamic = smaller number)
multiplied by your car's speed
multiplied by your car's speed again
multiplied by your car's speed a third time

Shortened even more, the amount of energy needed to push your car through the air is
(a bunch of stuff you can't control)
multiplied by your car's speed
multiplied by your car's speed
multiplied by your car's speed

--Peter

[Pete Dumbleton]

See the [b]BIG jump at 65 mph? We, as a country, could save a whole mess of gasoline and reduce our greenhouse gas emissions by a [b]HUGE amount by setting our metropolitan area (where the highest number of freeway miles are driven) speed limit to 50 or 55 mph and our rural area speed limits to 60 mph.

Oh, where is Nixon when we need him again?

[Chester Taje]

I 100% agree with reducing Highway speed limits, if for no other reason than to conserve fuel.

[Pete Dumbleton]

There will be disagreement from folks who want to get where they are going faster than others want to. Ultimately, the price of fuel will sort out the priorities. Those who want to get there faster can pay for it.

[peterh]

There will be disagreement from folks who want to get where they are going faster than others want to. Ultimately, the price of fuel will sort out the priorities. Those who want to get there faster can pay for it.

The "let the market decide" approach doesn't work well with traffic laws. One person driving 70 on two-lane highway where 55 is the norm is a hazard; ditto for someone driving 55 on where 70 is the norm. It's why we have speed (and other traffic) laws.

--Peter

[Pete Dumbleton]

Someone may come up with the idea of having a different speed limit for each lane, like many states effectively have for trucks and trailers right now, with special lanes and limits. The reality is that trailer pullers must drive at 55 on the same roads as folks at 70 right now.

I was just asked today to sign a petition in Washington for an initiative to "take back" the diamond or multi-passenger commuter lanes that were intended to get more vehicles off the roads. Right now, in the middle of all this fuel fuss.

Well, we used to have high speed limits and then we went back to 55 to save fuel and now we are back at high speed limits. I don't think we'll be going to 55 soon, and if we did, we'd eventually find ourselves at the high limits again, because complain as we might, fuel is still very affordable.

Here's an interesting chart I just found, comparing 1980 to 2008; look at gas increase compared to other increases (I have no idea if these two years are a good comparison, but there it is):

[Bruce H]

<span style="font-size:12pt;line-height:100%">I can give my experience pulling a 21' Bigfoot with three different trucks. The trailer weighs 5,500 lbs. loaded with water and supplies. Bigfoots have a flat front and they catch a lot of wind. </span>

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<span style="font-size:12pt;line-height:100%">The first truck was a 2003 Ford F-250 King Ranch with a 6.0 diesel engine and an automatic transmission. It was the cat's meow. It weighed 7,400 lbs full of fuel with me on board. I always write my odometer reading on my fuel ticket and figure the mileage of each tank. You will vary a gallon or two depending on which way the drive slopes at the gas pump but it averages out over several tanks. On the open highway with no trailer the truck would get 19 to 21 mpg. The 21 mpg was at 65 mph. Running 75 mph would reduce it to about 18 mpg. A 20 mph head wind would cut that down even at 60 mph. The heavy automatic transmission worked flawlessly. Pulling the trailer thousands of miles it got between 11 and 15 mpg. I seldom pulled the trailer any distance over 70 mph. </span>

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<span style="font-size:12pt;line-height:100%">I tried to economize on the next truck and bought a 2006 Chevrolet K-1500 with a 5.7 V8, automatic transmission, regular cab, short bed with trailer tow package. I got the truck new for $19,900.00. The first month I pulled the trailer on a 4,000 mile round trip to San Diego. The engine had plenty of power but it could not be used because the transmission would not hold high gear on the level on the highway. To maintain mid 60s mph I was running in third gear and turning the engine 2,700 to 2,800 rpm. I could not use the cruise control to run at those speeds because several times on slight up grades the engine would abruptly shift down to second gear and race the rpm up near 5,000. An application of more than 25% throttle resulted in a down shift and excessive engine speed. I stopped at Chevrolet garages twice and had them check the shift points of the transmission. They said it was within specs. I was worried about tearing the little truck up. It got about 16 mpg on the highway with no trailer and pulling the trailer it got between 8.5 and 11 mpg. That was in the low to mid 60 mph range and it also depended on wind speed and whether the gasoline contained ethanol. That truck was too light and short to safely handle that trailer on slick roads. I got caught in an ice storm in Oklahoma City and wound up camping at the Flying J for 17 hours waiting for the ice to melt. </span>

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<span style="font-size:12pt;line-height:100%">After 12 months of that I decided to go back to a three quarter ton with a standard transmission. I was tired of not being able to keep the transmission in the gear I wanted. The Chevy was a nice little truck still under warranty but the Chevrolet dealers absolutely did not want it on trade in. I stopped in at a Ford dealer to see what it would cost to order an F-350 single wheel one ton fixed the way I wanted with a standard transmission. They got on their dealer locater and found a truck exactly like I had specified. Then the guy said the magic words: "we would like to have your little Chevrolet". My Ford has a 5.4 liter (330 cu. in.) gas V8 and a 6 speed manual transmission. It will run out about 2,600 rpm at 65 mph in 5^th gear. It turns 2,000 to 2,100 rpm at 70 mph in 6^th gear. It is a little short on power pulling the trailer in high gear. I will not run WOT. I feather the throttle. If I have to give it more than 30 or 40% throttle to maintain speed I will down shift. Any up grade will require a down shift to 5^th gear. With no trailer and no head wind The truck gets 15 to 16 mpg on the highway burning gasoline with 10% ethanol. That is at 64 to 68 mph. In states such as New Mexico where the gasoline does not contain ethanol that mileage will increase 8 to 12%. It gets from 8.5 to 10 mpg pulling the trailer with speeds in the mid 60s and using gasoline with ethanol. The lower end of that mileage range is in a head wind. If I am faced with driving into a headwind of 30 or 40 mph I figure the best thing to do is camp until it quits blowing. When I get a tank of gasoline without the ethanol, mileage jumps to 12 mpg pulling the trailer at the same speeds with no head wind. I know this information from thousands of miles of pulling the trailer with both trucks and keeping my own records. </span>

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<span style="font-size:12pt;line-height:100%">Several Midwestern states have recently instituted laws that require all gasoline sold at retail contain 10% ethanol. Missouri is one of them. There is no sticker on the pump telling the purchaser what he is getting. Oklahoma is a mixed bag with many of the stations selling ethanol and again there is no information at the pump about the ethanol content. </span>