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Old 05-15-2014, 05:49 PM   #1
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Best MPH to get highest MPG

Lets say that 8 mpg was the average, just for this conversation. How much would that change by driving 65 MPH as opposed to 55 or 75mph. What is the perfect speed to cruise at in order to get the best MPG? Would speed change it more than 1 mpg, or does it really not make that much difference at speeds most of us drive?

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Old 05-15-2014, 05:56 PM   #2
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If you were to reduce speed to the point you just stay in your highest gear without lugging the engine you'd get your best mileage. Of course you'll get run off the road trying to go that slow on any main highways or interstates, but you'll pass more fuel stops that way! The difference between ~55 mph and 65-70 mph is dramatic, air resistance becomes the big robber at higher speeds.


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Old 05-15-2014, 06:00 PM   #3
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I was told by a fellow who used to work for Peterbuilt (shop foreman) that the best place to run a diesel was at the RPM where the torque peaks.

On my F450 it seemed to work out.
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Old 05-15-2014, 06:06 PM   #4
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Car, truck, or any motorhome. Gas or diesel, I would think around 50 to 55 mph will give the best mph. Depending on wind and terrain, your millage will very. Ours makes between 4 and 7 and I never check it anymore. It's got a fuel gauge, and a miles to empty and I drive by these. I did check it last month going to Arkansas. Fighting terrible winds from home to Tulsa the first day it got a tick over five. Pulling toad on trailer with no genny running. But I was running it pretty hard at 65 to 70. Just gas up and use your card, and don't look are dwell on the price. You can't take it with you.
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Old 05-15-2014, 06:09 PM   #5
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Since I pull a toad that should not pulled over 65 MPH I have gotten used to about 62 MPH. My 02 Dolphin 5355 shifts into overdrive at 55. It is a comfortable speed.
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Old 05-15-2014, 06:12 PM   #6
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The shortest answer is, you have to do the distance runs and the math to figure out what works for your individual coach and driving style.

Lots of variables involved. Calculate it over several similar trips on the same or similar terrain, speeds and load. Try to make it mixed driving -- avoid calculating just exit-to-exit on the freeway; you'll probably come out with artificially high milage. Driving at high altitudes in a gasser will also net measurably lower mpg than at sea level.

All told most class A's tend to get 7 to 9 mpg all around, depending on load, road conditions, and driving style. Hasn't changed much even with the conversion from carbs to fuel injections on gassers, and diesels tend to be put in heavier chassis/coach bodies so it mostly evens out.

Try to keep it at 55-60, and you'll see better milage than going 65-75 with the rest of the car traffic. Cruise control is helpful on relatively flat roads and rolling hills, but turn it off for better milage in steeper terrain; you can slow down on the uphill runs, take it easy on the engine, and pick up (safe) speeds downgrade. Your driving habits can have a significant impact.

If you pick 8 mpg at 60, for your example, you'll drop it to 7 mpg or less fairly quick as you pass 70 to 75. Wind resistance has a lot to do with it; the faster you push the "brick" down the highway, the more you have to overcome the air resistance. That's why some trucking companies set a speed limiter in their fleets to 67. Much more than 65 (60, really), and you loose in fuel costs versus what little time you might make up.
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Old 05-15-2014, 06:45 PM   #7
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Truck Driver's Rule of Thumb

one mile per hour equals .1 gallons per mile. So 10 miles per hour faster equals 1 mile per gallon less. It is just a rule of thumb, but it works fairly well. My son in law drove over the road for about 10 years and always finished in the top 5 drivers for anybody he drove fo,r when it came to fuel mileage. Two of his biggest "tricks" were 1) drive like you have an egg between your foot and the accelerator pedal, and 2) never let it idle, shut it off if you weren't moving. The other thing is researching your particular engine and find where the peak hp and or torque rpm is. Driving at peak is key to good fuel mileage. This information is provided by the engine manufacturers.
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Old 05-15-2014, 07:14 PM   #8
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My Coach Seems to be an Anomoly

I have a 2004 with Workhorse/Allison. All the wisdom from the Forums said that 62 MPH was the "Sweet Spot" for this combination so stayed at that range for the first year. Next year, took a long trip that involved a lot of driving on flat roads. Using scan gauge, tried extended periods at 62 and then at 67. Picked up almost 1 mpg at the higher speed. What was really surprising was that when I drafted tractor trailers at what I felt was a safe distance at the legal speed of 65, I actually picked up 2 mpg and easily exceeded average mpg of 10 while pulling my toad. I guess every rig is different.
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Old 05-15-2014, 07:18 PM   #9
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With our Ford V10, we get the best mpg in the 62-65mph range. Granted there are dozens of variables that can & do effect that, but over the past 9 seasons with our coach, that's what we've discovered.

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Old 05-15-2014, 10:11 PM   #10
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I've found that I " enjoy the view at 62 ".....gets me what I feel is decent for the circumstances fuel mileage, gives you more time to react, most pass you and you do less lane changing.
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Old 05-15-2014, 10:26 PM   #11
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I've recorded every gallon of gas I put in my coach since the day it was new. It now has 67,000 miles on it and the average mpg (hand calculated) over 67,000 miles is 7.1.

I drive at the speed dictated by the road and traffic conditions, very seldom over 65mph.
In my driving time on long flat stretches, think crossing Nevada on I80, I've run at 65mph for five miles and then 55 for five miles and watched the mpg calculator. Didn't notice any appreciable difference.

As far as I'm concerned 55 to 65 are about the same, over that I don't know.
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Old 05-16-2014, 02:00 AM   #12
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The cat 3126 in my coach has the best torque at 1650 rpm, thats 57 mph. I average about 10.5 mpg. If I go 65mph the milage drops a bunch...
Dave and Laura & two cats
02 Discovery with Accord toad
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Old 05-16-2014, 06:28 AM   #13
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I have an 05 5355 Dolphin on a Workhorse chassis. I have found over the years that runing at 2300RPM witch is about 62MPH gets me the best fuel milage. Towing my car I get an average of about 7.8MPG and without the toad it's 8.6.
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Old 05-16-2014, 06:45 AM   #14
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This paper by Caterpillar discusses how road speed, RV weight, wind speed, etc, affect engine power usage and, therefore, mpg for diesel-powered MHs. It doesn't matter if your engine is a CAT or a Cummins, the relationships are the same: Understanding RV/Coach Performance

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