Gas mileage/Fuel consumption determination

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Gas mileage/Fuel consumption determination

June 09, 2017, 01:36:36 pm

I'm curious how other owners determine their fuel consumption. My reason for this is that I have detected a difference of up to 1.0mpg between the gas mileage indicated at the dash display and what I calculate after filling.

At each fill, I record and reset the gas mileage at the dash display. I also record the miles traveled from the trip meter and the odometer, and how much gas I put in to fill the tank. Then I divide the miles traveled by the fuel added to completely fill the tank. It is interesting to me that there is often a significant discrepancy between the dash display for the mpg and the calculated value. The calculated value is true. On our last 2500 mile trip, towing an enclosed motorcycle trailer, my best tank was 9.8mpg and my worst was 6.9mpg. The dash display was always different than the calculated value, always higher, and at worst, off by 1.0mpg. Factors that affected the mileage the most were average speed, stop and go, headwinds, and terrain. 55mph average yields the best fuel consumption, but 62mph average is not significantly worse. It amazed me how much a headwind decreases the mpg.

One of the reasons this is interesting to me is that the indicated dash display range in miles to empty is likely determined by multiplying the dash display mpg times the amount of fuel remaining. Since the dash display mpg on my rig is not accurate and often overly optimistic, then I might run out of fuel when the dash display for the range indicates that I have many more miles to empty.

Naturally, I aim fill between when the fuel tank gauge shows 1/3 and 1/4 tank full. I also estimate this in miles, based on the mileage at the last fill. But in a pinch, it would be nice to be able to rely on the dash range value. As an aside, I make the same observations and notes for my motorcycle (OCD?) and the values are very close. The range indicated on the motorcycle dash is accurate within a few miles with a range of 220 miles. Why not the E450?

My wise Dad used to tell me "A man who has two watches never really knows what time it is."

How do you calculate your mileage (for those who care about such things) and is your dash display close to calculated reality?

Harold

Re: Gas mileage/Fuel consumption determination

Reply #1 – June 09, 2017, 01:40:44 pm

I was on an extended motorcycle ride with another guy whom I hadn't ridden with previously. After several fuel stops and watching me record and calculate in my logbook, he finally asked "What are you writing?"

I answered "Poetry."

He seemed satisfied and it took me a lot less time than explaining and justifying.

HD

Re: Gas mileage/Fuel consumption determination

Reply #2 – June 09, 2017, 02:15:11 pm

Wow, you have a computer in your LD?

I track mine manually, on a spreadsheet I keep in my phone. Total odometer reading minus last full, divided by gallons purchased. I keep a running total since fill ups vary by a gallon or two depending on the am mount that I squeeze into the filler neck.

On a new vehicle I have the computer. It is fairly accurate to my calculations. But on an older vehicle my computer is off by at least 1 MPG. I always calculate my own. Habit.

Re: Gas mileage/Fuel consumption determination

Reply #3 – June 09, 2017, 04:53:12 pm

Hi Harold,

"I'm curious how other owners determine their fuel consumption. My reason for this is that I have detected a difference of up to 1.0mpg between the gas mileage indicated at the dash display and what I calculate after filling"

We have tracked our rig's mpg in a small book since we picked it up 13 years ago. Needless to say, there is no digital display/trip calculator on our rig. I calculate the old fashioned way, miles travelled divided by amount purchased. BUT even then, there is often a significant discrepancy from tank to tank. I attribute that to the variable of how full I fill the tank.

What I mean by "how full I fill the tank", is do I stop pumping when the pump clicks off the first time? Or do I continue past the 2nd click off? Or keep pumping? Depending on how sensitive the pump is to clicking off, I often end up adding several more gallons. I am fairly inconsistent with that.

At the end of a long trip, I usually add up all the fuel purchased and divide the total trip miles to arrive at a trip MPG. That seems to negate the tank to tank variable of how full i got the tank. And that trip MPG has been fairly consistent over the 130,000 miles we have put on our rig.

For what it is worth, I try to NEVER let my fuel gage go below 1/4 tank remaining. Knowing how many miles to empty would be absolutely useless to me. I have also been told by a Ford dealer to NOT overfill the tank; that it could be harmful to the charcoal cannister.

My new Chevy pickup has a very accurate calculator. It helps that on this truck, when the pump clicks off, it is as full as I can get it. So the fill ups are consistent. And I can readily see the difference in MPG whether I am towing or not.

Steve K (wondering how the gas companies know when we are loading the rig for a trip and raise the gas prices?)

Re: Gas mileage/Fuel consumption determination

Reply #4 – June 09, 2017, 05:08:29 pm

Thanks Steve,
That is good information, especially that your average mpg has been consistent over the past 130K miles.

Overfilling a tank can definitely put liquid gasoline in the carbon canister, which will cause the vehicle to stop running. This has been a problem with newer motorcycles, particularly enduro bikes that have carbon canisters to meet emission standards. Topping off the tank and then running over rough terrain is a good recipe for a stalled engine, encouraging some owners to remove their carbon canisters. I never thought about it with cars and trucks, but it seems like a reasonable assumption.

I did discover on a previous Ford truck that had two tanks that if you run a tank out of gas a few times, it ruins the fuel pump. The tech explained that the gas in the fuel pump lubricates and cools it and running it out of fuel is very bad for the pump. Besides, who wants to run dry with only one tank?

HD

Re: Gas mileage/Fuel consumption determination

Reply #5 – June 09, 2017, 05:30:31 pm

Do you use the generator much? The generator will consume gas from your tank bypassing the fuel delivery monitored by the engine computer. The result will be a true reading of engine fuel mileage on the mpg meter, but that doesn't take into account the gas you replace that the generator used.

Steve

Re: Gas mileage/Fuel consumption determination

Reply #6 – June 09, 2017, 06:00:53 pm

Quote from: Blueox25 - June 09, 2017, 01:40:44 pm

I was on an extended motorcycle ride with another guy whom I hadn't ridden with previously. After several fuel stops and watching me record and calculate in my logbook, he finally asked "What are you writing?"

I answered "Poetry."

He seemed satisfied and it took me a lot less time than explaining and justifying.

HD

Harold, I noticed you writing in your logbook during your visit to our area. I didn't inquire but figured that's what you were doing.

I've heard similar statements on the Subaru forums regarding dash gas mileage vs the hand calculated method. The general consensus seems to be that the hand calculated way is most accurate. I haven't driven the Outback enough to verify it yet but hope to do so in the future.

Re: Gas mileage/Fuel consumption determination

Reply #7 – June 09, 2017, 06:17:17 pm

Greg,
I noticed you noticing. I didn't want to tell you about the poetry lest you might ask me to do a reading...

It is still just OCD.

HD

Re: Gas mileage/Fuel consumption determination

Reply #8 – June 10, 2017, 07:27:16 pm

I do not have a Ford computer in my '08. I added an UltraGauge. Once I did the recommended calibration procedure, it has been within 1 or 2 tenths of a gallon when I fill, adding usually around 40 gallons. I figure that is pretty accurate. I have compared its trip mileage with my GPS routing and with Google Maps, and on a long haul, the UltraGauge is more accurate than the vehicle's odometer. It reports MPG and miles to empty based on the above information, and I trust it.

Ken F in WY

Re: Gas mileage/Fuel consumption determination

Reply #9 – June 11, 2017, 12:16:09 am

Sounds good Ken. Ultra gauge well may be in my future. Sounds like it may be a good way to monitor transmission temp. as well.

HD

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