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Old 10-05-2022, 05:49 AM   #1
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Gen fuel consumption

I’ve almost always ran my generator while traveling. We’re usually traveling in summer or warm locations in winter so I run the A/C. I have a 10k Onan Gen. My rig usually gets between 6-7 MPG. Yesterday I ran about 400 miles without the Gen running I used a significantly less amount of fuel. I’ve always assumed the Gen used about a gallon per hour of fuel. Does anyone know the actual consumption rate ?

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Old 10-05-2022, 06:06 AM   #2
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I have heard 1/2 gal per hour quoted a lot but I don’t know how that figure was arrived at.

I suppose the electrical load (how many ACs) would affect the consumption so I’m not sure one guy’s data would jive with another.
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Old 10-05-2022, 09:26 AM   #3
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Old 10-05-2022, 09:35 AM   #4
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The manual on your particular model generator will tell you the normal 1/2 and full load consumption rates.

Ken
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Old 10-05-2022, 09:54 AM   #5
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If it took 8 hours to travel 400 miles, the generator would have probably used 4-6 gallons of fuel over that time period.

Other factor to consider are:
Driving speed
Headwind/tailwind
Fuel blend, B-20 v #2
When the pump shut off on the before and after fill ups
Topping off the tank on either fill up

Approximately what amount of fuel was “significantly less” on your recent trip where you did not run the generator?
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Old 10-05-2022, 10:17 AM   #6
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Originally Posted by glsponchos View Post
I’ve almost always ran my generator while traveling. We’re usually traveling in summer or warm locations in winter so I run the A/C. I have a 10k Onan Gen. My rig usually gets between 6-7 MPG. Yesterday I ran about 400 miles without the Gen running I used a significantly less amount of fuel. I’ve always assumed the Gen used about a gallon per hour of fuel. Does anyone know the actual consumption rate ?

Thanks
We use our generator very frequently and run 1 or 2 (15K BTU) A/C units and notice very little change in MPG, even on long trips.

Ours is newer than yours is so it may be different but according to the manual it burns in Gallons Per Hour:

No Load: .33
Half Load: .54
Full Load: .78

Hope this helps!
Sam
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Old 10-05-2022, 12:14 PM   #7
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I've calculated the difference in mileage with and without the 10kw generator running. What I came up with is an average of 0.9gph running. That's figured over 49,000 miles of driving and over 600 hours on the gen. I plug that into a spreadsheet that calculates MPG between fillups with the number of hours the generator runs. It comes out almost exactly the same as when running between fillups without the generator.
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Old 10-05-2022, 01:07 PM   #8
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I've calculated the difference in mileage with and without the 10kw generator running. What I came up with is an average of 0.9gph running. That's figured over 49,000 miles of driving and over 600 hours on the gen. I plug that into a spreadsheet that calculates MPG between fillups with the number of hours the generator runs. It comes out almost exactly the same as when running between fillups without the generator.
WOW! I too notice little, to now difference but your method is much more scientific.

Sam
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Old 10-05-2022, 04:12 PM   #9
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I've calculated the difference in mileage with and without the 10kw generator running. What I came up with is an average of 0.9gph running. That's figured over 49,000 miles of driving and over 600 hours on the gen. I plug that into a spreadsheet that calculates MPG between fillups with the number of hours the generator runs. It comes out almost exactly the same as when running between fillups without the generator.
I bet if you plugged in 0.6gph you’d get “almost exactly the same...” as well.

A difference of 0.3 gallons per hour when you’re burning 8-9gph just to move the coach is almost statistically insignificant.
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Old 10-05-2022, 07:04 PM   #10
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I bet if you plugged in 0.6gph you’d get “almost exactly the same...” as well.

A difference of 0.3 gallons per hour when you’re burning 8-9gph just to move the coach is almost statistically insignificant.
Exactly. So as an example, let's say you normally get 6mpg at 60mph and the genny uses .6gph just to make the math easy. In an hour you'll go 60 miles and burn 10 gallons of diesel in the engine, and another .6 gallons in the genny. 10.6/10=1.06, or a 6% difference in fuel consumption. So you'd burn 6.36 gallons of diesel instead of 6. More realistically it's probably less for most because even *we* go faster than 60 and we normally see about 7.5 mpg.

This ignores what happens if you're running the cab air to stay cool instead of the genny plus the roof air. I have no fuel consumption data handy for the engine-driven AC compressor, but it definitely takes more fuel to use it than not to use it, and the 80' round trip for its refrigerant does not improve its efficiency.

This is almost a rounding error IMO. Wind is likely a bigger variable most of the time.
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Old 10-05-2022, 07:40 PM   #11
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Exactly. So as an example, let's say you normally get 6mpg at 60mph and the genny uses .6gph just to make the math easy. In an hour you'll go 60 miles and burn 10 gallons of diesel in the engine, and another .6 gallons in the genny. 10.6/10=1.06, or a 6% difference in fuel consumption. So you'd burn 6.36 gallons of diesel instead of 6. More realistically it's probably less for most because even *we* go faster than 60 and we normally see about 7.5 mpg.

This ignores what happens if you're running the cab air to stay cool instead of the genny plus the roof air. I have no fuel consumption data handy for the engine-driven AC compressor, but it definitely takes more fuel to use it than not to use it, and the 80' round trip for its refrigerant does not improve its efficiency.

This is almost a rounding error IMO. Wind is likely a bigger variable most of the time.
Agree; a strong head wind and/or stop and go traffic are much more detrimental to fuel mileage. However, I'm impressed Lt Dan took such a scientific approach.

Sam
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Old 10-05-2022, 08:08 PM   #12
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Agree; a strong head wind and/or stop and go traffic are much more detrimental to fuel mileage. However, I'm impressed Lt Dan took such a scientific approach.

Sam
Just to be clear...in no way was I attempting to disagree with what Dan posted.
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Old 10-06-2022, 06:48 AM   #13
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Thanks for everyone’s feedback Will have to study this more. Obviously one data point is not enough Thanks
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