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Old 11-16-2014, 07:23 AM   #1
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Speed vs MPG

Just made the trip south for the winter. Most states on the east coast now have upped their speed limits to 70mph. That is too fast to tow.

Who tows at the new higher speed limit?

For my rig 2012 F-450 towing a 15,000lb. 5th wheel I get 9.5mpg towing at 70 - 72 mph. And 10.6 - 11mpg towing at 55 to 57 mph. I typically tow at 62 - 65 mph and get around 10mpg.

Oh...I have Michelin XPS Ribs on my trailer.

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Old 11-16-2014, 11:43 AM   #2
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The speed limited on I-20 and I-10 in west Texas is 75 MPH most of the way, but 80 MPH in far west Texas. That's now the limits for both cars and trucks, night or day, towing or not. Which means the ticket limits on the rural interstates are now 79 MPH most everywhere in Texas, and 84 MPH in far west Texas.

Most 18-wheelers cruise at 70 to 75 MPH, but there are some that go on up to 84 MPH. Most one ton dullies towing gooseneck equipment trailers hauling oil field supplies cruise at the ticket limit.

But I know that speed costs MPG. On my F-250 with 7.3L diesel engine, MPG when towing my 8,000-pound mid-profile 5er was
12 MPG @ 62
11 MPG @ 66
10 MPG @ 70
9 MPG @ 75
8 MPG at the ticket limit of 79 MPG

Slowing down to less than 62 MPH resulted in worse MPG (because of HP and torque curves). So I almost always cruised at 62 when towing.

The newer TT and F-150 is harder to pin down. Speed costs MPG, but I haven't figured out exactly how much. The lie-o-meter in my Lariat shows about the same 9 to 10 MPG (on premium gas) whether towing at 62 or 70. But because the trailer tires are speed rated to 65 MPH, I usually tow the TT at about 65.

Grumpy ole man with over 50 years towing experience. Now my heaviest trailer is a 7,000-pound enclosed cargo trailer, RV is a 5,600 pound Skyline Nomad Joey 196S, and my tow vehicle is a 2012 F-150 EcoBoost SuperCrew.
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Old 11-16-2014, 12:06 PM   #3
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Air drag is the biggest spender when talking about fuel economy. Air drag goes up with the square of speed, so the drag at 60 isn't twice what it is at 30... it's 4 times more, and at 90, it's 9 times more.
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Old 11-16-2014, 07:31 PM   #4
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I run between 62 and 65 mph on the interstates. The sweet spot on my engine is between 1900 and 2000 RPM That is 62 to 65 mph. It pulls most hills in overdrive at that rpm without downshifting, I might drop to 55 while pulling the hill, let the transmission downshift when speed drops to 52, EGT's approach 1300*, or the water temperatures start rising above 220*. This gives me between 10 and 11mpg grossing between 24,000 and 25,000#.
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Old 11-17-2014, 09:47 PM   #5
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Technically, if you're using ST trailer tires then your speed is limited, by the tires, to 65 mph. I have Michelin Rib LT tires and am not limited by the 65 rule. However, I run around 70 max for mpg reasons; faster and I lose mpg. But I don't seem to get any better mpg by going slower. I've towed in CA at their 55 mph limit and my mpg is about the same as at 70 mph. Bring some headwind into the equation and my mpg goes in the toilet.
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Old 11-18-2014, 05:27 AM   #6
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All I know is that I generally drive 55-60 with my class A and I see a lot of folks both towing and class A's pass me...

Over 60 MPH as the speed goes up the MPG Drops like a rock.
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Old 11-20-2014, 09:42 AM   #7
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Noticed the higher limits too,I hate getting run over,worse is a large discrepancy in speeds .
i found myself towing 5 mph faster now -60 65 is now 65 70,mileage is down noticeably .
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Old 11-20-2014, 11:33 AM   #8
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Really, you are worried about MPG? What about safety and your lives? The critical consideration is your power curve. Too many people out there are flying down the road towing a trailer or a toad and they are at the upper limits of their power curve--in other words, if they had a blow out and needed to put the pedal to the medal to put power to the drive wheels to keep you going straight so you can avoid losing control, they have nothing left to give. Who want's to risk their vehicles and their lives trying to recover at 75 -80 mph as opposed to 55-65? It just makes no sense at all.
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Old 11-20-2014, 11:51 AM   #9
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I agree with Tetonchief, I feel my best speed for both gas mileage and safety is around 60-62 mph. I have had people pulling trailers drive past me at 70mph +(speed towing at the time is 55) and their trailers are swaying all over the place. I have never understood what the hurry is when you are on vacation and why that would be more important than your safety. So you get to your destination an hour sooner???I think alot of us are retired so there is really no reason to hurry. The journey is half the fun and usually more rewarding than the destination.
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Old 11-20-2014, 12:25 PM   #10
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You can travel across PA now and save as much as 6.4 minutes. The heck with the fuel and tires, full speed ahead! Life is so much better with all that time saved.

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Old 11-20-2014, 07:58 PM   #11
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Made a living towing trailers up to 22k GVW so I'm comfortable running up to 70 mph. I don't push my personnel feel good speed limits on other folks.

I've used lots of different one ton DRW trucks with gas and diesel so each truck was different in speed/mpg figures. LOL ...some of those old 454/460 gazzers ran in the 4-6 mpg range at just about any speed.

I don't haul much anymore so the Dodge/Cummins in my sig does all the RV towing. I've noticed best mpgs (12-13) are in the 55 mph range. Lower mph doesn't increase this trucks mpg pulling my 11200 lb 5er. I've ran 65-68 mph all day long trip after trip and generally mpg drop to the 10-11 range all depending on head winds.

Speed limits here in OK on the interstates are 75 mph and our goofy C word legislature is talking about raising those speeds to 80-85 mph on some areas.
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Old 12-01-2014, 10:44 AM   #12
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Fleet truck operators have been teaching their drivers to plan their trips. If they need to get to their destination to pick up or drop off a load in 5 hours and it is 300 miles away then driving at any speed faster than 60 MPH is going to waste fuel and provide no advantage. When the driver knows that they are more inclined to stay at this speed. The same applies to towing a trailer for recreational use. I go 300 miles or less on any given day for most trips. I try to avoid long periods sitting behind the wheel. For that distance driving at 62-65 MPH I will arrive at my next destination in 5 hours or less. If I drive at 70 MPH I will burn 10% more fuel and save 30 minutes or less driving time. Seldom does saving 30 minutes in these circumstances make a difference. Whether I arrive at 3:00 or 3:30 in the afternoon at a campground is not important. Air drag increases with the square of your speed. My truck when towing or hauling will get 13-14 MPG at 62-65 MPH. At 75 MPH the MPG drops to 11 MPG and add in a headwind and the truck is getting 9-10 MPG. Whatever time I might save is offset by having to make more fuel stops.
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Old 12-01-2014, 11:00 AM   #13
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Originally Posted by Tetonchief View Post
Really, you are worried about MPG? What about safety and your lives? The critical consideration is your power curve. Too many people out there are flying down the road towing a trailer or a toad and they are at the upper limits of their power curve--in other words, if they had a blow out and needed to put the pedal to the medal to put power to the drive wheels to keep you going straight so you can avoid losing control, they have nothing left to give. Who want's to risk their vehicles and their lives trying to recover at 75 -80 mph as opposed to 55-65? It just makes no sense at all.
To Say nothing of the fact class A tires that I know about are speed rated at 65 mph.
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Old 12-01-2014, 11:17 AM   #14
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Well, I guess it all depends on what your time is worth.
Some people bill $100+ per hour for their time. Saving 2-3 miles per gallon will never make you whole.
Even if your time is only worth $15/hr, you can easily afford the extra gallon of fuel if you save 30+ minutes.
Just another view from an economics angle.

Also, what can you do with the time saved? Sleep in, or arrive at camp before dark or rain? Play with the kids... That's worth something too.

Saving 2 or 3 mpg really isn't ever much of a consideration for me. I do like to get the best fuel economy I can... up to a point.

While the air resistance does go up exponentially with speed, there is also a dynamic efficiency that is specific to each tow vehicle related to gearing and engine. I know my truck is more efficient at 70mph than 60mph, even when towing. Above 72mph it falls off sharply.

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