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Old 09-13-2017, 10:53 AM   #1
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Best Mileage Speed

I am new to diesel engines. What is the speed to get the best MPG? Being new to diesel, what do I need to know for using and maintaining my engine and fuel?
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Old 09-13-2017, 11:02 AM   #2
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I don't try to squeeze every drop of diesel. However, I find that at 65mph I get between 7 1/2- 7 3/4 mpg. You'll need to check your mileage at different speeds. There are instruments that you can attach to give you current and average MPG. Scan Gauge D is the one I use.
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Old 09-13-2017, 11:26 AM   #3
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Cat produced this file on RV diesel performance . Cummins has a similar file but I don't have a copy , but from what I read of it the info was the same .

Personally , my coach shifts into 6th gear at 58 mph , and I set cruise when possible at 62 to help keep the trans from downshifting , with trans controller in Economy mode.
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File Type: pdf Cat_RV_Performance.pdf (246.3 KB, 89 views)
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Old 09-13-2017, 11:30 AM   #4
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We have the little 6.7L Cummins, I have found for our rig 63 MPH is the sweet spot. Reasonable fuel economy, 8.5 mpg and high enough in the torque range to pull hills without down shifting.
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Old 09-13-2017, 01:24 PM   #5
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I have seen 62mph often discussed. I use 62mph and it does seem a sweet spot.
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Old 09-13-2017, 01:30 PM   #6
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The lowest speed that will hold the highest gear is going to get you the best mileage.
But I expect that's going to be annoying slow for you and everyone around you.
To know for SURE you'll need a device to measure fuel consumption for you, but I expect you'll find it's in the low to mid 60s.
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Old 09-13-2017, 01:34 PM   #7
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Cummins - with your engine serial number you can contact Cummins and they will tell you a suggested sweet spot for your engine. Ours was recommended at 1650 rpm.

We drove very close the recommended for the majority of the trip. Included hills of Virginia as well as northern Ontario.

On a 6000+ mile trip we averaged 7.6 mpg according to the fuel log which included generator and aquahot use. We boon docked over half of the days we were out. The computer showed an average of 8.7.
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Old 09-13-2017, 06:24 PM   #8
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About 50 would give you better mileage than 60. And 60 would give you better mileage than 70. But, between 60 and 65 seems the best compramise. I typicall run about 63 or there abouts. Past 65 mileage seems to drop off pretty rapidly.
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Old 09-13-2017, 06:38 PM   #9
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The charts on the Cummins site show 1370 as the best RPM for my ISX, that's just over 60 mph as 1330 is 60.
7.6 mpg over 47,000 miles.
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Old 09-13-2017, 06:53 PM   #10
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Up hill, down hill, tail wind/head wind have the most impact on MPG. Otherwise, I tend to agree that over time, highest gear at lowest RPM seems like the best strategy....Depending on rig and tranny, eg, Allison 3000, low 60's seems to work and keep tranny from repeatedly down shifting. The primary keys to a happy diesel are three: 1-clean fuel, 2-clean air, and 3-clean oil.
So depending on your rig/engine manufacturer, new fuel filter[s] every year, preferably a brand name; change oil and filter every year [name brand for both]; and last, a new air filter every couple years [2 or 3], again brand name. Opinions vary but INHO, brand names offer the "potential" for a better product. Avoid the "mom/pop" fuel stops, use major brand stations with high volume customer flow which will turn-over fuel stock more frequently, so less opportunity for fuel contamination....these are the basics....
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Old 09-13-2017, 07:03 PM   #11
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In summary:

Lowest speed for highest gear for best mpg (for me above 58 mpg). I normally run at 62 mph.

Change your engine oil and filter, fuel filters annually.

Air filter every 2 years.

Allison tranny oil and filters with TranSynd every 5 years

Air brake dryer filters every 3 years.

Note: Above assumptions are for those who put less than 10K mi on their units each year.
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Old 09-13-2017, 07:12 PM   #12
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Quote:
Originally Posted by Gordon Dewald View Post
Cummins - with your engine serial number you can contact Cummins and they will tell you a suggested sweet spot for your engine. Ours was recommended at 1650 rpm.

We drove very close the recommended for the majority of the trip. Included hills of Virginia as well as northern Ontario.

On a 6000+ mile trip we averaged 7.6 mpg according to the fuel log which included generator and aquahot use. We boon docked over half of the days we were out. The computer showed an average of 8.7.


There is a formula for calculating an engines "sweet spot". However this definition of sweet spot isn't related to fuel economy. It seems to be the rpm where the hp and torque curve cross. Here is the formula:

HP X 33,000 = A
Torque X 6.283 = B
A/B = sweet spot rpm

The Cat white paper addresses the fuel economy question and my reading of it supports the idea, as has been posted, that the best economy is achieved at the lowest rpm that will reliably hold top gear.
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Old 09-13-2017, 10:01 PM   #13
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I would think It would be under 60mph, possibly even under 55 mph where best fuel economy is. Always thought aerodynamic drag sucked more fuel then being able to use the highest gear....

My Jeep is that way, 45-50mph seems to be the best for fuel economy, going 60+ is not good and the faster I go the faster it sucks it down. My gas RV had similar results, better mpg in 5th at lower speeds however that's not practical on the highway.

Seems like most go low 60's in DP's due to being able to use the highest gear.
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Old 09-14-2017, 02:57 AM   #14
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Doing the math, my sweet spot would be 1900 RPM. I went 300 + miles at 65 MPH and got 7.
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