Adjusting mpg for generator use

[Urbanhermit]

Our 8.5 genset, per manual, burns 1 gph at full load.

Say we drive four hours at 65 mph running the generator at full load, and then take on 40 gallons of fuel. (Parameters chosen for clarity)

It would seem that we'd gotten 6.5 mpg (260 miles / 40 gallons) But if we deduct the four gallons the generator used and use 36 gallons for the engine, we see we've gotten 7.2 mpg.

(Okay -- headwinds, uphill, pulling a TOWD.)

If you're using the generator just to run the front AC, as we sometimes do, generator consumption is probably more like 0.5 gph. I didn't use that in the original parameters because its not as predictable as the manual's figure for full load.

[Old-Biscuit]

When fuel gauge indicates 1/2 full start looking for fuel station.
*Gen fuel pickup tube is at 1/4 tank and GEN will stop


SO......use the 6.5 mpg for calculating travel distance

[Scepter40]

Interesting. I generally just calculate the overall MPG as a total consumed between the systems. If we do a cold weather trip, we may run the Aquahot for the furnace while traveling. I can pretty much guarantee a 6MPG number no matter what, so that's what I go off of. Fuel gauge is erratic, so I generally go off of miles traveled to figure out when and where I get fuel.

[d23haynes57]

Originally Posted by Urbanhermit

Our 8.5 genset, per manual, burns 1 gph at full load.

Say we drive four hours at 65 mph running the generator at full load, and then take on 40 gallons of fuel. (Parameters chosen for clarity)

It would seem that we'd gotten 6.5 mpg (260 miles / 40 gallons) But if we deduct the four gallons the generator used and use 36 gallons for the engine, we see we've gotten 7.2 mpg.

(Okay -- headwinds, uphill, pulling a TOWD.)

If you're using the generator just to run the front AC, as we sometimes do, generator consumption is probably more like 0.5 gph. I didn't use that in the original parameters because its not as predictable as the manual's figure for full load.

For these comparisons I like to use 60 as the baseline. 60 mph, 60 minutes/hour etc.

So for an hour, 60 miles, even at 1 gph, the gallons per mile become .017.
If the engine is getting 8 mpg at that speed, that becomes .133 gallons per mile.
The generator can add about 12.5% to the fuel consumption, cost for the trip.

[Urbanhermit]

My gauge is very consistent and seems accurate. It's marked in eighths. 75 gallon tank. The gauge is off while I burn through the volume of the filler pipe (always top up to get an anal-retentive data set) but by the time it shows 3/4 full I can begin using 9.4 gallons an indicated eighth. I keep a fuel consumption log and have a little calculator Velcroed to the steering wheel. With these I can do a quick calculation of MPG and remaining range, though I use 70 gallons rather than 75 to be on the safe side. Only when I've been too interested in scenery do I let the gauge get below 3/8 full before looking for a truck stop. I've trained myself to be content with 63 mph max and lately have been cruising at 58 to 60 (had a huge decal on the back reading SPEED GOVERNED BY -----FLATION which got me occasional waves and honks and people coming up behind me, hanging there long enough for a picture to be taken (?) before passing, and an occasional finger). Two or three hours on the slab and a bit of stop-and-go has brought my consumption average up from low 6es to high 7s.

[twinboat]

Using 1/2 ,or worse yet, full load on a 8500 watt generator sounds way off. Between 1 air conditioner and maybe some battery charging, your only going to be drawing closer to 1/4 load, or 2000 watts.

[Spdracr39]

It's a pretty insignificant difference no matter how you cipher it so I don't even bother. I plan on filling between half an a quarter which gives me plenty of wiggle room to look for the best prices in the area I'm traveling in at the time.

[d23haynes57]

If I'm going to run my generator while driving for AC, I'm going to turn them all on and let them do their thing. The cost is what it is, $1 to $4 per hour. I just prefer not to hear or feel it running while driving. I used to go crazy thinking or fuel economy even for my cars. At some point especially far from home, it can burn all the fuel it wants to do what I want or need it to do as that means I am not pushing it or waiting for a tow truck.
When camping without hook ups it becomes how long can I stay before I need fuel, propane, or water?

[bigb56]

Beware of some models that don't actually use the float for displaying fuel level during normal operation, they use the ECM to calculate things like "miles till empty" and on some, like the Sprinter chassis, to position the gauge. The ECM does not measure fuel taken by after market add ons like generators and diesel heaters which could, and has, resulted in running out of fuel while the gauge still shows fuel remaining. The float is only used to reconcile the total when refueling.

Even though the generator on the Sprinter is very stingy, the fuel tank is also very small.

[DarkSideJeep]

I have a 150 gallon fuel tank, 3/4 of a tank is about 112 gallons. If I calculate 6mpg that would be 675 miles. In practice I get much better than 6mpg especially when I remember to turn on the economy mode. I adhere to the speed limit or 70mph whichever is lower. My bottom line is I would never drive 675 miles in a day so I just full up near my destination each day. The most fuel I have ever taken was 80 gallons after two-day drive, when I knew that the next day would be a short drive where I did not fill because I had enough fuel to get to my destination. I do run the generator for the AC or Aqua Hot one thing to remember is that the generator seldom runs at full load because you seldom have everything running and using power all at the same time. Even if you have everything "ON" The AC or heat and the Refrigerator only use power when their thermostats say they need to.

[Urbanhermit]

Originally Posted by twinboat

Using 1/2 ,or worse yet, full load on a 8500 watt generator sounds way off. Between 1 air conditioner and maybe some battery charging, your only going to be drawing closer to 1/4 load, or 2000 watts.

Then my hypothetical adjusted for 1/4 load might be a consumption of around 0.5 gph on a pure guess, reasoning that an 8.5 kW would burn at least 0.25 an hour without a load, and the adjusted combined mpg would be 6.8 mpg -- theoretically. The data I used, as noted, is hypothetical. The principle stands.

A bit scary to know ECM-driven fuel gauges are made optimistic by whatever the peripherals burn (I assume my '06 uses a float or a flow-through meter) and emphasize the wisdom of fueling early. There is a correlative principle in sailing: "The time to reef the sail is when you first think about it."