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Old 12-14-2012, 08:05 PM   #1
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Surprising Fuel Savings at 55 mph

On our last roadtrip, I miscalculated the distance to our next fuel stop and decided to slow down a little to save gas. (Not as bad as it sounds, since we carry 5+ gallons in a marine tank to refuel our generators, which can be used for emergencies.)

When I slowed from 60 to 55 mph, I reset the computer on our 2008 Tundra CrewMax (5.7L, 2WD) to see if this would help any.

Before resetting the computer, we were getting 12.2 mpg. After slowing to 55 mph, the computer reading at the next gas stop indicated that we had gotten 15.2 mpg over the previous 150 miles (at 55). And, this average held steady over the next 100 miles to our next stopover. I expected a slight improvement, but 3 mpg was a real surprise.

I really don't want to drive slower, but a 24% improvement in fuel economy is making me reconsider our cruising speed. And, that 5 mph didn't seem to make much difference in the traffic passing us.

Is anyone else driving 55 to save fuel?
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Old 12-14-2012, 08:11 PM   #2
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On our last roadtrip, I miscalculated the distance to our next fuel stop and decided to slow down a little to save gas. (Not as bad as it sounds, since we carry 5+ gallons in a marine tank to refuel our generators, which can be used for emergencies.)

When I slowed from 60 to 55 mph, I reset the computer on our 2008 Tundra CrewMax (5.7L, 2WD) to see if this would help any.

Before resetting the computer, we were getting 12.2 mpg. After slowing to 55 mph, the computer reading at the next gas stop indicated that we had gotten 15.2 mpg over the previous 150 miles (at 55). And, this average held steady over the next 100 miles to our next stopover. I expected a slight improvement, but 3 mpg was a real surprise.

I really don't want to drive slower, but a 24% improvement in fuel economy is making me reconsider our cruising speed. And, that 5 mph didn't seem to make much difference in the traffic passing us.

Is anyone else driving 55 to save fuel?
A great addition to a tow vehicle, is an "Air speed indicator".

If it reads higher than the speedo, then slow down, when towing. If it reads lower than the speedo, then you can increase the towing speed.

You will find that it adds additional mileage to what you have already experienced.

Andy
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Old 12-14-2012, 08:19 PM   #3
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It's the wind resistance that does not increase proportionally with increased speed. It becomes a larger and denser wall to push at higher speeds.

I like 60-65 mph but drop down if heading into the wind to save fuel (money).

doug k
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Old 12-14-2012, 08:39 PM   #4
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Driving 55 mph

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... Is anyone else driving 55 to save fuel?
We routinely tow at 55 mph. There is the fuel savings to consider, as well as the safety factors. I feel much more in control at 55 mph, can react to emergencies better, and can slow down or stop in shorter distances. We have never had a tire failure, and I believe that towing at the slower speed is a factor here.
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Old 12-14-2012, 08:47 PM   #5
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Doug is correct. I usually drive 63, but if I have a lot of time, I'll back it off. Usually the rule of thumb for a solo vehicle is 10% improvement by reducing 5 mph.....so, I guess with the additional frontal area, rear suction and weight, your findings don't surprise me too awful much.
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Old 12-14-2012, 08:58 PM   #6
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It gets even better at 45. Like the time I forgot to fill up at McDermitt, NV and coasted into Winnemucca on the last remnants of diesel fumes in my truck and nursing it along at sub 40 mph speeds . But then, wow, it takes forever to get anywhere. That is a major reason I don't visit Ca towing.

Sometimes I go for mileage and sometimes I just want to get where ever it is I am heading. Like from Tucson to the Rim, who cares what the mileage is, just get us out of the freakin oven and into some cooler temps.
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Old 12-14-2012, 11:07 PM   #7
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Surprising Fuel Savings at 55 mph

Fifty-Five miles per hour is my normal cruising speed when towing either my Airstream, Argosy, or car hauler. With this as a known, I set my travel day accordingly and the trip becomes a time to relax and enjoy the changing scenery. I find that with the Suburban, towing the Overlander reduces my fuel economy by 2 MPG over the solo fuel economy cruising at 65 MPH; the Minuet only reduces the fuel economy by 1 MPG when compared to the solo fuel economy cruising at 65 MPH. I rarely travel Interstates while towing as I don't find them conducive to vacationing and relaxing so much of the time, the posted speed limit maxes out at 55 MPH.

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Old 12-15-2012, 06:50 AM   #8
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From an undergraduate physics class, there's a change in the nature of aerodynamic drag on a vehicle that occurs around 60 mph. Up to the change, the drag has a linear relationship to speed, above the break point the drag changes to a square relation. Below the breakpoint the drag increases in proportion to speed and above it the drag increases in proportion to the speed squared (goes way up with a little increase in speed). That's exactly why Pres Carter lowered the national speed limit to 55 mph back in the 70's during the oil shortage, because slowing to that speed uses a significantly lower amount of fuel.
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Old 12-15-2012, 07:22 AM   #9
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Thumbs up 55-58

We tow with an 06 8.1 Burb. 15960lb combined wt.

55-58mph steady cruise, 2500mi trip got 11.1-12.2 mpg.

65mph over an 800mi trip, 8.6-9.2mpg.

Small brain conclusion.....55-58.

Bob
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Old 12-15-2012, 07:24 AM   #10
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On our last rally from Valley of Fire to White Sands, NM our caravan needed to travel at 45mph due to posted limits and the fact our wagon master didn't want to lose anyone. Got 22mpg!
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Old 12-15-2012, 08:34 AM   #11
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That's exactly why Pres Carter lowered the national speed limit to 55 mph back in the 70's during the oil shortage, because slowing to that speed uses a significantly lower amount of fuel.
I was still in school when President Nixon signed the double nickel law in 1974. Interesting reading how the mileage savings weren't so good -- many states wouldn't enforce the 55 limit as tightly as they had at higher speeds. So yes, it certainly does come down to actual ground speed.

I'm still mulling over Andy's airspeed indicator. This could get positively Rube Goldbergian...
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Old 12-15-2012, 09:01 AM   #12
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So that is how we got 14.5 mpg traveling 750 miles down to the Great Smokey Mountains np and back on the Blue Ridge Parkway. The BRP speed limit is 45 mph.

Dan
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Old 12-15-2012, 09:07 AM   #13
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We are 17000# combined. I tow between 55-58. Yes it takes forever to get there but we use lots less fuel, feel more in control, and the kids see more...
The faster you drive the bigger the parachute.
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Old 12-15-2012, 09:41 AM   #14
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Is anyone else driving 55 to save fuel?
Not to save fuel, but I've done what you did because we had some time to get to our destination and got the results you mentioned - 15+ mpg! Tundra 5.7L 2WD.

Coming back from Death Valley a few years back when gas prices were almost $7 in DV, I slowed down to 55 on my Goldwing motorcycle and got 56 mpg!
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Old 12-15-2012, 09:41 AM   #15
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I was still in school when President Nixon signed the double nickel law in 1974. Interesting reading how the mileage savings weren't so good -- many states wouldn't enforce the 55 limit as tightly as they had at higher speeds. So yes, it certainly does come down to actual ground speed.

I'm still mulling over Andy's airspeed indicator. This could get positively Rube Goldbergian...
Bob.

I used the Airspeed indicator for 4 years, when I was with Caravanner Insurance.

In spite of guesses, opinions, and gut feelings, it's use made a huge difference in fuel mileage.

Little do we suspect but as we travel across a given area, the wind speeds do change as well as from different directions.

It's used also very clearly demonstrated what happens to the air currents when meeting an oncoming vehicle. That quickly explained why people sometimes lose control of their rig, because of that increase in wind speed, especially when unexpected.

Airspeed indicators are not really that expensive, nor difficult to install. Certainly, even with a moderate amount of travel per year, the use of one, more than pays for itself in short order.

It also quickly teaches the driver of how quickly something that we cannot see changes, which really suggests changing our driving habits.

Most folks are not aware that if you meet an oncoming vehicle that is going, as an example 60 miles per hour, that speed momentarily adds to yours, and then oscillates downward. If not rigged and adjusted properly hitch wise etc, you could be in very sudden trouble.

I call it's use, "a guide to efficient and safer towing".

Andy
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Old 12-15-2012, 09:52 AM   #16
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Better believe it! Twenty-five years ago my father in law had an experience pulling a 19' SOB with a '70s Buick sled into the teeth of a gale in SE Colorado. He got 4 mpg amidst some very tough (and not very fast) driving that day. They found that renting snowbird accommodations in AZ & TX was just about the same $$ as camping and sold the trailer after that trip.

He'd always speak about how we could pick up an inexpensive trailer in those locations from people who didn't want to drive it back home. Airstreams? Nope, no Airstreams among them... Guess I just had the bug.

BTW - lack of hills and not using cruise control are a great contributor to better mileage.
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Old 12-15-2012, 10:44 AM   #17
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But it does seem to help fuel milage in the mountains to cost up in speed going downhill and let the speed drop going uphill. On my diesel if I set the cruise at 55 it is a real pain in the hills because it slows down just before it starts up again.
I went to the west coast and back in a Volvo pulling a pop up back in the double nickle days. I do not know what it did to the mpg, but it sure took a long time. 60-63 on level ground is actually a good number with my current rig. My biggest deterent to mpg is stopping frequently. Each rest area seems to drop the mpg meter 3 mpg or so and it takes a long stretch for it to build back up. Also uses a lot of fuel in the campground parking.
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Old 12-15-2012, 10:47 AM   #18
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Starts and stops are a real killer for me too! Once I get going, I try not to stop until I need fuel, or it gets close to quitting time.
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Old 12-15-2012, 05:40 PM   #19
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Quote:
Originally Posted by Crusty View Post
From an undergraduate physics class, there's a change in the nature of aerodynamic drag on a vehicle that occurs around 60 mph. Up to the change, the drag has a linear relationship to speed, above the break point the drag changes to a square relation. Below the breakpoint the drag increases in proportion to speed and above it the drag increases in proportion to the speed squared (goes way up with a little increase in speed). That's exactly why Pres Carter lowered the national speed limit to 55 mph back in the 70's during the oil shortage, because slowing to that speed uses a significantly lower amount of fuel.

Not to be too picky on this, well maybe, there isn't any change in the "nature of aerodynamic drag" the power to overcome aerodynamic drag (not aero drag itself) is a cube relationship to the increase in speed. What happens is that the power to overcome mechanical losses is basicly linear to speed. So at lower speeds these mechanical losses are the major part of the total power requirement to push any vehicle down the road. You can see that as the speed increases, with mechanical losses increasing only linearly and aero increasing as the cube of the speed increase there is indeed a point that aero is much more significant and this is where you see the increase in fuel consumption.

Ref: Bosch Automotive Handbook, 3rd Edition pages 324-326.
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Old 12-15-2012, 07:17 PM   #20
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I just experience this today. It was the first day of our 3200 mile trip to Florida. We drove all day in a horrible rainstorm (Northern Ca.). While crusing down I5 I glanced at the computer and saw 9.2MPG. I looked at the speed and it was 62. The flow on I5 is 72, so I was hesitant to get too many guys up my tailpipe. Anyway, I slowed to 55 and the computer jumped to 11.3MPG. Of course, this was a crude measure just using the current MPG gauge, but I think the concept was pretty clear.

I think we were also into a headwind with the rain too. I sure like the idea of a airspeed gauge.
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