EcoDiesel, A Maiden Voyage

[Monster5601]

On the topic of mileage, why do the on board computers calculate the milage wrong. In the 5.7l hemi, screen says 19.2 mpg or so for last 3 fill up, but real mileage has been 17, 17.04, and 17.8 mpg. Not complaining about mileage, just wondering. Does the computer not count idle fuel use?

I assume you are using your average MPG numbers. Reset your average MPG at each fill-up (or use one of the trip meters) since the average is calculate over time and not between fill-ups. There has been a lot of flack about how average MPG are calculated, so much so, newer vehicles are starting to use average MPG over the last 50 miles traveled.

Each time you fill-up there is no way to tell how close to full you are or if you filled up to the same level as last time. Temperature of fuel determines the expansion of the gasoline, about 1% for each 12 degrees F, the warmer the gasoline, the more used to do the same amount of work. Also the amount of energy in the fuel you bought, some blends use more or less ethanol than others. While each of these (and other factors) don't add up to much, they are accumulative and impact the average MPG over time.

I have a 2013 RAM (5.7 L Hemi) that replaced my 2003 Tahoe (5.3L) and both vehicle's average MPG aligned with my hand calculations. I did reset the averages on both vehicle around the time our area switched between summer/winter blends because of the significant change in MPG between the two blends.

 

[LD1]

Each time you fill-up there is no way to tell how close to full you are or if you filled up to the same level as last time.

You can get close.

I use the same pump everytime filling my car. And once it kicks off, thats it. I dont try to squeeze any more in. But my car dont have the idiot computer so all I can do is hand calculate.

But in general, I agree that they could be more accurate. So far, the chevy I had with the 5.3L and both Hemi's (03 and 05) have been spot on.

My diesel is way off though. We made a trip down to newport aquarium just into kentucky. It is right off the freeway. Prior to getting on the freeway, I filled up. It was 280 miles there and back to the truck stop I filled up at. ALL freeway driving. Computer said 21.1 MPG. So it should have only taken 13.27 gallons to fill her up. But took just over 15 gallons. For a hand-figured 18.5mpg.

Now I could live with being a percent or two off. Or even 5% off. But 14%????

And it is always like that. I always hand figure ad it comes out a few mpg less. Even towing the trailer and showing 13-14 on the computer, by hand it is 11-12.

Must be to make people who never bother to figure by hand feel good.

 

[QueBota]

2001.5 with Edge EZ and BD Stage II injectors. Caused overhead display to be off by approx. 4 mpg.

I drove the truck for about 20K miles stock, EZ and injectors added approx. 1.5 mpg, hand calculated.

During the warmer months my overhead reads around 24 mpg per tankful, hand calculated it normally
comes out to around 20 mpg. Can't complain, truck weighs in around 7800 lbs with me and a full tank
of fuel.

Q

Ahh, yeah, I've heard the chips mess up the MPG meter. Did your chip have any real effect on fuel economy? You hear all the stories about 5-6 more MPG, but I have a hard time believing that and think it's just folks using the gauge and not hand calcs (or rounding 16 up to 20 to make them feel better ).

 

[fordmantpw]

2001.5 with Edge EZ and BD Stage II injectors. Caused overhead display to be off by approx. 4 mpg.

I drove the truck for about 20K miles stock, EZ and injectors added approx. 1.5 mpg, hand calculated.

During the warmer months my overhead reads around 24 mpg per tankful, hand calculated it normally
comes out to around 20 mpg. Can't complain, truck weighs in around 7800 lbs with me and a full tank
of fuel.

Q

Thanks for the info. That sounds reasonable for a gain with a chip and injectors. As I expected, the chip is throwing the gauges off drastically, making people think they are much better off than actual.

 

[DarkBlack]

Each time you fill-up there is no way to tell how close to full you are or if you filled up to the same level as last time. Temperature of fuel determines the expansion of the gasoline, about 1% for each 12 degrees F, the warmer the gasoline, the more used to do the same amount of work. Also the amount of energy in the fuel you bought, some blends use more or less ethanol than others. While each of these (and other factors) don't add up to much, they are accumulative and impact the average MPG over time.

I have a 2013 RAM (5.7 L Hemi) that replaced my 2003 Tahoe (5.3L) and both vehicle's average MPG aligned with my hand calculations. I did reset the averages on both vehicle around the time our area switched between summer/winter blends because of the significant change in MPG between the two blends.

Remember, you can't confuse that day's air temperature with the pump gas temp.
I think you'll find the gas comes out of the big ground tanks within a pretty small temp range. You're paying for the volume of gas ( and filling your tank) as it comes from the ground not the outside air temp.

 

[dfkrug]

On the topic of mileage, why do the on board computers
calculate the milage wrong?

I read somewhere that the computer counts the total ON time for the pulses sent to all the fuel
injectors, then divides it into the odometer mileage over some period. The computer is
calibrated for the stock injectors, so the quantity of fuel for the measurement period is
known, unless you change the injectors. The measurement period can be so short that
it is effectively instantaneous. Hence the readout you see on the dashboard.

Anyone heard different on how the MPG is calculated?