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Old 08-20-2017, 08:40 PM   #1
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Calculating Mpg question

Hi,

I like to keep tabs of my mpg just so I know my range.

I have a Scan gauge E but not sure how accurate it is as have not configured it when topping up.

Usually I divide the mileage done by the amount of gallons I have used.

Is the fuel shut off different on different gas pumps?

Thanks,
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Old 08-20-2017, 10:36 PM   #2
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howdy - my scangauge used to be fairly accurate on my Chevy Express van, not so much now on my Sprinter, but it does give you a good idea on how economical you driving habits are... Regarding when nozzles shut off - this is based on flow restriction with fuel coming out, or more correctly, back into the nozzle. Overall they all seem to work about the same. Not enough variation to matter, unless the nozzle length is different. Math is number of miles since last fill-up divided by number of gallons used for that distance gives you miles (traveled) per gallon used. ST
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Old 08-20-2017, 11:03 PM   #3
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It takes many thousands of miles to get an accurate fuel mileage. I'm getting 7.6 over 47,000 miles. On the flats my Silverleaf shows 13 to 15 mpg and on a hill around 3 mpg. Wish I did more flatland driving.
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Old 08-21-2017, 12:17 AM   #4
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Quote:
Originally Posted by 1stMH View Post
Hi,

I like to keep tabs of my mpg just so I know my range.

I have a Scan gauge E but not sure how accurate it is as have not configured it when topping up.

Usually I divide the mileage done by the amount of gallons I have used.

Is the fuel shut off different on different gas pumps?

Thanks,
and you have to account for the fuel used by the generator if you run it.
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Old 08-21-2017, 04:16 AM   #5
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Thanks all for the replies,

I do only use the generator for maybe an hour or so a month so don't even bother adding that to my calculations.

I do it more so that I can arrive at my detonation with more than half a tank in case I do need the generator.
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Old 08-21-2017, 04:50 AM   #6
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Calculate your MPG by trip or a particular time frame.

When you fill-up the angle your motor-home is setting will affect how many gallons it will take to fill. This can vary considerably.

Start your trip with a full tank and zero miles on your odometer at the end of your trip or after several fill-ups do the arithmetic total miles divided by total gallons.
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Old 08-21-2017, 08:46 AM   #7
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Just a comment on using the generator while driving relative to determining your mpg. I think the following is one logical method. I am assuming 1 gal of fuel per hour of generator usage (this seems to be the most often used number), even during non-driving periods, and an average speed at about 60-61 mph, so consumption would be about 1 gal per hr. That amount would then be subtracted from the total gal consumed in order to calculate only the driving mpg usage.

For example: You took a trip of 365 miles starting with a full tank. The trip took 7 hours, with a 1 hour rest in the middle in which the motor is turned off but the generator was left running. Thus actual drive time was 6 hours and you ran your generator the whole 7 hours. You averaged between 60-62 mph when driving (confirmed by the calculation 365 mi/6 hr drive time = 60.8 mph). At the end of the trip you filled your tank with 55 gals. To calculate your actual driving mpg you must subtract 7 gals for generator usage, leaving 48 gals. The result for only the motor's fuel consumption would be 7.6 mpg (365 mi/48 gals = 7.6 mpg). Your nominal mpg would be 6.64 mpg (based on the raw calculation of total 365 miles/total 55 gals).

So, if you're obsessive you could set up a spreadsheet to make calculations easier, but you have to do some time-tracking on your own since GIGO (garbage in, garbage out) controls your results. Have fun.
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Old 08-21-2017, 09:16 AM   #8
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As noted, generator usage should be included in MPG as as well as idling time. We spend lots of time parking, putting the slides out and getting set up with the MH idling.
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Old 08-21-2017, 09:53 AM   #9
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If you're going to split hairs with fuel mileage, I.e. Idle time, generator use etc., don't forget to calculate the speedo factor if necessary.

My speedo is off by 4 mph at 65 mph. Verified by gps. Therefore my mileage is different than what the odometer says.
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Old 08-21-2017, 10:02 AM   #10
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My rig came with a Silverleaf 400 CL. It doesn't measure fuel usage by the generator or AquaHot, only what the engine uses so no mental gyrations needed.
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Old 08-21-2017, 10:17 AM   #11
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The best way to calculate gas mileage is when it gets to 1/2 tank fill up. Get to your destination set up relax and have a beer
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Old 08-21-2017, 01:38 PM   #12
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The best way to calculate gas mileage is when it gets to 1/2 tank fill up. Get to your destination set up relax and have a beer
Dcbenny, one doesn't need a spreadsheet for that! Cheers.

Worrying about getting an exact MPG would be low on our list of concerns were it to even be on the list. I just wanted to provide a starting point for some of those who are so concerned about how MPG is effected by their generator running while they travel down the road. Being aware of one's nominal or actual MPG is useful as a baseline in that if under normal conditions (taking into account winds, hill grades, temperature extremes, etc.) your MPG degrades enough to notice; then there may be a problem with the motor's operation. I know MPG will degrade if I have strong headwinds or climb steep grades, or it will slightly improve under the opposite conditions (funny how the bad conditions seem to effect MPG more than good conditions improve MPG), so I don't lose any beer-drinking time unless there's something on the coach that's very hot, smoking, or on fire while we're rolling or when we stop. Life's too short.
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Old 08-21-2017, 01:59 PM   #13
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Quote:
Originally Posted by 1stMH View Post
Hi,

I like to keep tabs of my mpg just so I know my range.

I have a Scan gauge E but not sure how accurate it is as have not configured it when topping up.

Usually I divide the mileage done by the amount of gallons I have used.

Is the fuel shut off different on different gas pumps?

Thanks,
My MH range is drive three hours then fill up, usually $100 at a dollar per litre. Repeat twice then park in campground. I use the Randy McNally GPS to enter fuel and it figures out I need to stop often.

Actually around 8 mpg normally. No idea how big the tank is, think 70- 80 gallons but I have never gone below 3/4 before I fill up.

My gague goes way past full so will not trust it.


Gas pumps for me depend on angle I am sitting on as my filler is 4-5 feet before it hits the tank. I need the nose down to get a good fill.
I could likely travel 500 miles before having to fill but I prefer more frequent smaller ($$) fill ups.
Terry
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Old 08-21-2017, 03:12 PM   #14
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Quote:
Originally Posted by 1stMH View Post
Hi,

I like to keep tabs of my mpg just so I know my range.

I have a Scan gauge E but not sure how accurate it is as have not configured it when topping up.

Usually I divide the mileage done by the amount of gallons I have used.

Is the fuel shut off different on different gas pumps?

Thanks,
My opinion is that with the volume of fuel we buy at once, a quart or two variance in fuel shut off won't matter. Do the math, for instance.

400 miles with 50 gallon fill up = 8 mpg
400 miles with 51 = 7.84
400 miles with 49 = 8.16

And that's with full gallon difference in shut off, which is unlikely. It don't matter

I have had pumps shut off early, so I do let it shut off once, then feed again slowly until it shuts off again.
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