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Old 04-04-2018, 02:56 PM   #1
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Gas Mileage versus speed

Greetings Folks,

Has anybody done a study of a typical Duramax diesel towing a load at certain speeds to gain the optimal gas mileage? It seems going 60 versus going 70 makes a big difference. Mighty obliged.....
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Old 04-04-2018, 03:08 PM   #2
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Cummins has, no idea about a Duramax though. Cummins has charts and their website used to have a calculator to help choose you rear axle gearing for best economy or performance. Mine showed 1380 rpm as most evonomical and I'm turning 1330 at 60.
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Old 04-04-2018, 03:11 PM   #3
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Slower will pretty much always gain you better results.
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Old 04-04-2018, 03:20 PM   #4
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You might find this interesting:
https://cumminsengines.com/uploads/d...whitepaper.pdf

The physics don't change.
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Old 04-04-2018, 03:25 PM   #5
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I'm betting you won't find many gas mileage tips for diesel engines.
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Old 04-10-2018, 08:07 PM   #6
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Quote:
Originally Posted by Bohdan View Post
Greetings Folks,

Has anybody done a study of a typical Duramax diesel towing a load at certain speeds to gain the optimal gas mileage? It seems going 60 versus going 70 makes a big difference. Mighty obliged.....
If only you had a Cummins, you'd have your answer.

It really boils down to the ideal RPM for maximum fuel efficiency. Your Duramax engine will probably be in that spot between 1600-1700 RPM's. That just a guess.
Speed will be determined by your gearing. Diesel fuel economy decreases almost exponentially as RPM's increases above the ideal range. Not sure what your setup is but it probably shouldn't be going more than 60-65 regardless.

If you have the vehicle and it has a setting for current MPG on the gauges, drive it around and see where you achieve the best economy. Real world testing will be more accurate that any other method.

Diesel car gets 35mpg at 80, 42mpg at 65.
Coach with toad gets 8.3mpg at 60-65.
Which I had the Duramax in the GMC 2500.
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Old 04-15-2018, 05:31 PM   #7
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Most vehicles get their best mileage at the speed that the automatic trans has just shifted into it's highest gear plus a couple of miles to keep it in that gear. A typical gas engine car would shift into 4th gear at 45, so 50 was the best mileage. Diesels like a little more RPMs so they don't lug, my 06 Duramax gets it's best mileage at 55-60 MPH.
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Old 04-15-2018, 05:34 PM   #8
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My Duramax in no wind and 65-70 mph gets me about 9 mpg. Winds, however, really robs MPG. I've gotten as low as 7 MPG in some pretty stiff winds. I tow a 19,000 lb toy hauler.
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Old 04-15-2018, 08:38 PM   #9
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My Duramax in no wind and 65-70 mph gets me about 9 mpg. Winds, however, really robs MPG. I've gotten as low as 7 MPG in some pretty stiff winds. I tow a 19,000 lb toy hauler.
...

My 06 Chevy w/FW weighs in at 25k lbs and I can get 11.5 MPG running 58-62 mph with little to no wind (especially on cruise control). I have gotten 23 mpg running bobtail with cruise set at 55 MPH. Driving habits can make a 2-3 mpg difference. I did get 7 mpg once hauling a flat faced 40' container on a gooseneck trailer in heavy head winds.
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Old 04-15-2018, 08:54 PM   #10
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The drag increase is Squared with the doubling of speed, this applies to every vehicle that moves thru the air. For speeds we drive the increase from 55mph to 65mph results in a 40% drag increase and about a 18% increase in power required to overcome it.


A really neat example of drag is the Bugatti Veyron. The car has 1,000HP and goes 252MPH flat out. It only uses about 140HP to make it to the first 125MPH but needs the other 860HP to double that and make it to 252MPH.
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Old 04-18-2018, 07:50 PM   #11
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I have a 2013 GMC Denali HD. We pull a 2011 Crossroads Seville & am about 23,000 total weight loaded for the winter.

Coming home from the southlands to Indiana, we filled up east of Ashville, topped off in Kodak, TN, then filled up again in Indiana. Pulling up I-75 averaging about 65mph, we got 11 mpg.

I try to stay between 1700-1800 rpm in 5th gear when towing just to keep the engine at a good rpm. 6th gear will let the engine lug and seems to cause more trans heat.

Just my results, everyone is a bit different.
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