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Old 12-13-2017, 08:26 AM   #43
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Pretty much always run mine while driving (Onan QD8000). Uses 1/3 of a gallon per hour. I like to keep the house cool while driving.
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Old 12-13-2017, 05:09 PM   #44
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Originally Posted by Ray,IN View Post
Generator use does NOT affect engine fuel mileage, it gets the same mileage whether the generator runs or not. The generator does affect total fuel used when both are running.


I agree, it would be difficult to get an accurate fuel mileage number if the generator were running for any length oh time. How would you know how much fuel the generator had used.
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Old 12-13-2017, 06:15 PM   #45
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Negligible.
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Old 12-13-2017, 08:19 PM   #46
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I agree, it would be difficult to get an accurate fuel mileage number if the generator were running for any length oh time....

Not difficult at all.

Get Silverleaf and you can monitor MPG all the time. It monitors the flow of fuel to the engine, accumulates the miles and calculates MPG. Simple.
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Old 12-13-2017, 08:25 PM   #47
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Ray, IN is correct. There's a lot of misunderstanding running through this thread. Your engine mpg remains totally unaffected by generator use. What IS affected is your total fuel consumed. To stay accurate you have to separate generator fuel from engine fuel.

We run our 7.5 Onan quite often when traveling. I figure it at .6 gph. I know that my Dutch Star gets 7.4 mpg. At every refuel I note both the generator Hobbs and the odometer. I've created a spreadsheet that accounts for "Generator Fuel Used" and "Net Fuel to Engine". Over 5.5 years it has proven to be quite accurate.
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Old 12-14-2017, 09:11 AM   #48
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About 1/4 per gallon of gas per hour if your not using the AC.
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Old 12-14-2017, 09:27 AM   #49
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Not difficult at all.

Get Silverleaf and you can monitor MPG all the time. It monitors the flow of fuel to the engine, accumulates the miles and calculates MPG. Simple.
Yep, my 7.6 mpg over 49,000 miles comes from my VMS 400 CL.
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Old 12-14-2017, 11:58 AM   #50
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change of pace... diesel coaches getting ~8mpg?? lol
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Old 12-14-2017, 01:53 PM   #51
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change of pace... diesel coaches getting ~8mpg?? lol
What's the funny part, you don't believe it? I'm at 8.3mpg after 37,000 miles with the 8.9L 400hp ISL. I hear it gets better after 50,000.
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Old 12-14-2017, 02:02 PM   #52
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7.5 to 8 mpg is a very real MPG number for the typical DP. About the same for the gassers. Our Mandalay has averaged 7.4 mpg for 97,000 miles.
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Old 12-14-2017, 03:47 PM   #53
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Ray, IN is correct. There's a lot of misunderstanding running through this thread. Your engine mpg remains totally unaffected by generator use. What IS affected is your total fuel consumed. To stay accurate you have to separate generator fuel from engine fuel.

We run our 7.5 Onan quite often when traveling. I figure it at .6 gph. I know that my Dutch Star gets 7.4 mpg. At every refuel I note both the generator Hobbs and the odometer. I've created a spreadsheet that accounts for "Generator Fuel Used" and "Net Fuel to Engine". Over 5.5 years it has proven to be quite accurate.
Yes, using the generator does not affect the amount of fuel the engine uses to propel the coach down the road. The OP's coach has a diesel engine and diesel generator which are both fueled by the same tank. If the coach travels 64 miles in one hour at 8mpg the engine will have used 8 gallons. With the generator running at 1/2 load, in this case about 30A which would require either both AC units to be on the entire time or 1 AC and the battery charger for the full hour, an additional 1/2 gallon will be consumed. This would effectively reduce the MPG to 7.5 because you will have now used 8.5 gallons while traveling 64 miles.

Tons of variables based on speed, traffic, grade, wind, driving habits, electric load, etc. For this type of coach the "effective MPG" will drop from hardly noticeable to 1MPG, with hardly noticeable being the more common observation.
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Old 12-14-2017, 04:37 PM   #54
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One of the questions in the OP's original post was:

"Just curious to know how much does running the generator while driving effects fuel mileage"

Fuel mileage will be affected if the generator uses the same fuel as the coach. Fuel mileage is affected by amount of fuel consumed, not fuel type, generator size, or brand. Generator efficiency would be a factor if it consumes less fuel per watt generated.

On a 50 mile trip that takes one hour to complete and your generator uses .5 gallons per hour:

If your rig gets 15 mpg with out generator it will get 13 mpg with it.
If your rig gets 5 mpg with out generator it will get 4.76 mpg with it.
If your rig gets 3 mpg with out generator it will get 2.9 mpg with it.

If your generator is only affecting mileage by .1 mpg I would assume it is only consuming about .1 gallons per hour. I am guessing you get better than 3 mpg without the AC(s) running.
Read my previous post.

The OP owns a 2003 Alpine Coach with 2 roof top AC units, either 13.5K or 15K and an Onan 7.5K diesel generator. I own a 2005. Based on the known fuel consumption rates of our generator and known AC loads in our coach it is possible to estimate the amount of fuel used by the generator and apply that data to determine the total amount of fuel used to travel a certain distance.

Gas coach, gas generator, diesel coach, diesel generator, 4K genset, 20K genset, 17mpg Sprinter, 6mpg Marathon, 80°, 110°, Florida, Colorado, everything will be different.

We get 8mpg, generator sips fuel. Running it while driving is a great way to exercise it if you don't want to run it at a campground or in your driveway or storage after a month long trip where it was never used.
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Old 12-14-2017, 04:39 PM   #55
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I agree, it would be difficult to get an accurate fuel mileage number if the generator were running for any length oh time. How would you know how much fuel the generator had used.
You can establish a baseline on the generator if you use it a lot. I once filled up immediately before and after a race event. We used the generator to run all 3 AC's for 96 hours straight. It used 48 gal or 0.5 GPH. That about as close as you can get to any accuracy.
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Old 12-14-2017, 04:40 PM   #56
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Originally Posted by mike brez View Post
We run ours all the time. Has no affect on mpg.
How can it not affect fuel economy when it’s using fuel from the same tank
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