MPG calculation ?

[Sharky]

the truck MPG shows it gets 21.5 mpg. took a 110 mile trip & used trip #2, it showed 24.6 mpg.
later in the week I took another trip to the same place but did not use trip #2 however the MPG for trip #1 did not increase. I thought the MPG for trip #1 should increase a tenth or 2 ???? what gives ?

Sponsored

 

[Msfitoy]

They're all off...I would do a manual MPG check with miles divided by fuel input...

 

[Frenchy]

I found it best to reset both at the same time and compare. I reset it all every time I fill the tank and that includes the Trip Meter on my GPS.

 

[harringtondav]

I take the same 110 mile trip to our river house regularly. Mileage has varied between 18-30 mpg. The extremes are strong head or tail winds. But any wind from any direction will cause fluctuations. Same with extreme cold OAT.
To get a 'true' highway mpg I wait until I'm up to 70 mph and reset to cancel out the stop and go getting up to speed.
On the rare zero wind days the mileage shown on either gage is the same. ....just south of 25 mpg.
I know the only true mileage is calculated manually. I use the above as a comparison of normal to spot any possible engine issues.

 

[MechanicalBob]

the truck MPG shows it gets 21.5 mpg. took a 110 mile trip & used trip #2, it showed 24.6 mpg.
later in the week I took another trip to the same place but did not use trip #2 however the MPG for trip #1 did not increase. I thought the MPG for trip #1 should increase a tenth or 2 ???? what gives ?

It depends on how many miles are on the "trip" you're calling your truck's MPG (assuming Trip 1). If it has a million miles on it to get 21.5 mpg, then one trip of 110 miles isn't going to move the needle.

 

[_TraMor]

What kills me is the ECU knows both how much fuel is flowing in AND how far you've gone with a pretty decent amount of accuracy. So, why are they all off? [Reaches for tinfoil hat]

 

[Dgc333]

What kills me is the ECU knows both how much fuel is flowing in AND how far you've gone with a pretty decent amount of accuracy. So, why are they all off? [Reaches for tinfoil hat]

The trip computer calculates fuel used by the duty cycle of the fuel injectors. Error is introduced by manufacturing variations in the injectors and the pressure variations generated by the fuel pump. Miles driven is calculated by a constant in the computer for tire diameter and counting the number of revolutions. Error is introduced by tires wearing and air pressure variations in the tires changing the diameter. I also suspect that the constant for the tire diameter is not adjusted for every possible tire brand that may be used. I know there was about a 2% difference in the constant in the computer for the tires on my Tremor verse what General listed for the tire.

Even using hand calculations there is error introduced. It is nearly impossible to fill the tank to exactly the same level everytime. Fill the tank more one time then on the next fill not quite as much and your hand calculation will show higher MPG and vice a versa. The distance will also have the same errors mentioned above.

If you record the trip display mileage and hand calculation mileage over at least a couple of thousand miles you can calculate the error between the two with reasonable accuracy. There is a sticky on this forum that describes how to go into the engineering screen on your truck and adjust an offset number to make the trip display and hand calculations agree more closely. I did this to my truck and previous Mustangs and consistently have the two agree within +/- .5 mpg. The degree of variation is then going to be determined by how much effort you place on filling the tank consistently.

 

[5gss]

the truck MPG shows it gets 21.5 mpg. took a 110 mile trip & used trip #2, it showed 24.6 mpg.
later in the week I took another trip to the same place but did not use trip #2 however the MPG for trip #1 did not increase. I thought the MPG for trip #1 should increase a tenth or 2 ???? what gives ?

If you didn't reset Trip #1, it's averaging fuel economy over a longer period, diluting the impact of your 24.6 mpg trip.

 

[TxOTRRanger]

I just let the computer handle it, I suck at math anyway so the simple solution is just trust what the truck tells me. On average over a three day period, which is about all I can get out of what I believe is a sixteen gallon tank in my truck. This is weather permitting mind you, wind, rain, etc. It averages anywhere from 23 mpg to about 27 mpg, after the first three days of this week it averaged 24.3 mpg with 98,145 miles on it at nearly four years old. This is a combination of city and highway driving, mostly highway.

 

[TxOTRRanger]

I'm going to update this, with the addition of the new wheels and all terrain tires, and the truck now has 103,000 + miles on it. The fuel mileage took a little bit of a hit considering the fact that the ATs are little bit more heavy duty than the previous HTs it had on it. Currently it averages, city and highway combination. Lowest average is 20 to 21.5 mpg. Highest average is 23 mpg, still roughly over a three day period. I declare it "Good Enough!" Lol

 

[Chris M]

I'm going to update this, with the addition of the new wheels and all terrain tires, and the truck now has 103,000 + miles on it. The fuel mileage took a little bit of a hit considering the fact that the ATs are little bit more heavy duty than the previous HTs it had on it. Currently it averages, city and highway combination. Lowest average is 20 to 21.5 mpg. Highest average is 23 mpg, still roughly over a three day period. I declare it "Good Enough!" Lol

Good to be satisfied with your choices. Good job!

 

[TxOTRRanger]

Good to be satisfied with your choices. Good job!

Thank you Chris, and thank you for looking out for me. I appreciate it

 

[TxOTRRanger]

Let quote myself here lol!

I'm going to update this, with the addition of the new wheels and all terrain tires, and the truck now has 103,000 + miles on it. The fuel mileage took a little bit of a hit considering the fact that the ATs are little bit more heavy duty than the previous HTs it had on it. Currently it averages, city and highway combination. Lowest average is 20 to 21.5 mpg. Highest average is 23 mpg, still roughly over a three day period. I declare it "Good Enough!" Lol

I took this after topping it off with fuel and making the 50 mile drive home.

Consistent

Sponsored