MPG onboard calculator

[Castrosua]

So most of you dont care about MPG's which is perfectly ok. (now smiles per gallon different story ) Hell, I got a few classic Continentals, that I do drive quite a bit.

But I do like to keep track of MPG's and just about everything else in all my cars, in maintenance binders as well as computerized form (excel and word files)

Anyways, back to my main question. How is MPG calculated onboard? I figured it was an estimate and was expecting it to be within a percentage of real MPG but....



So I do this for all my cars, document mileage among maintenance actions, any modifications if any, oil consumption tire condition and placement. Its just a good practice to set baselines as far as consumption so if any time numbers dont match up, I can start looking into things. It helps on cars without ODB or computers. So dont jump on me with the whole "get a prius if you care about mpgs )

Also some of you that were in the Air Force working with aircraft, yes thats where the 781F comes from. I was a crew chief, so its how I modeled my car books and digital records, from 781 forms.

So now that all that is out of the way. There is a discrepancy between real world numbers and displayed mpg. The max variation Ive seen is about 4.69 percent to as close as to 0.734 percent to real world. I cant see a noticable pattern, so the only thing I can imagine is that if this is GPS derived then weather, signal, and various errors in how position is calculated that make up the error Ive seen so far.

So, anyone noticed this or cared? I know Im kind of powergeeking it here rolled up in a blanket of severe OCD, but I am curious what sort of numbers you guys and gals have seen.

Al

[Castrosua]

Sorry for the double thread. My computer froze. So if nobody answers to this one or the other, could one of you delete this, or if replies are split, merge them?

Sorry for being a dummy. I dont know why my computer showed an error when i hit submit. Also, if yuo do merge this post doesnt need to show up so it doesnt interrupt any interesting replies. Thanks!

[rebelyell22]

distance traveled divided by fuel used...

[Merc]

44000 kilometers on Pumpkin fuel log never re-set 14.5 litres for every 100 km's.

[Mr_Draco]

As you said, it is just an estimation and in my experience, it has always been high. The computer doesn't know exactly how much gas is left or how much was used.

Also keep in mind, not all gas pumps are the same too. You pay for what comes out of the pump but depending on the gas station, 1/4 to 1/2 gallon of gas that you paid for is still in the hose.

[Castrosua]

I didnt think of that, the gas in the hose. I just thought it was interesting to compare. I always log mileage and always do complete fill ups. On all my cars, and I might be one of those rare people that use the trip odometer. It can get a bit tedious keeping records on 4/5 cars (well 4 and a quarter since the 5th is a parts car and its pretty much just a shell now, but hey, got a title and keys to it lol)

Anyway, I just like knowing as much as I can about my cars, especially when they are new and I havent had a chance to get intimate with em hehe.... I didnt know squat about my Crown Vic until I got a parts car that I tore apart. No I KNOW em inside and out. In 10 years when Ill be ripping my 5 (if I still have it) then Ill be a Camaro expert too!

[Red2014SS]

Originally Posted by Mr_Draco

As you said, it is just an estimation and in my experience, it has always been high. The computer doesn't know exactly how much gas is left or how much was used.

Also keep in mind, not all gas pumps are the same too. You pay for what comes out of the pump but depending on the gas station, 1/4 to 1/2 gallon of gas that you paid for is still in the hose.

Yeah, but when you first begin to fill up, there is also a 1/4 to 1/2 gallon in that same hose that you didn't pay for. It all evens out in the end.

[Chevy_cowboy]

Interesting data, I track fuel mileage the same way but never track what the DIC shows, always hand calculated numbers.

I'd guess your variance is due to a combination of variance in gas pump calibration, and evaporation in the car.

Originally Posted by Red2014SS

Yeah, but when you first begin to fill up, there is also a 1/4 to 1/2 gallon in that same hose that you didn't pay for. It all evens out in the end.

Unless you follow someone that kicked off the pump and then drained every drop out of the hose before hanging it back up. Not many do that, but some do.

[Splitter]

Hand calculated is the way to go.

Onboard DIC adjusts your MPG every second the car is on an operating, if you idle for one minute, it will factor in one full minute of 0 MPGs

[SSJ455]

I use a spreadsheet for my cars as well showing mileage, mods, new tires, oil changes, etc. I never go by what the DIC says, in fact I couldn't tell you how close it is to my spreadsheet because I don't look at it.

[ZED SLED]

Originally Posted by Comrando

What was that movie were the guys neighbor bought a VW Bettle and was always calculating his MPG. And the neighbor would sneak over in the middle of the night and dump a gallon or two of fuel in his tank?

Sgt. Hacker did that to Sgt. Carter's car on Gomer Pyle.

[kmarshall2121]

Originally Posted by Mr_Draco

As you said, it is just an estimation and in my experience, it has always been high. The computer doesn't know exactly how much gas is left or how much was used.

Also keep in mind, not all gas pumps are the same too. You pay for what comes out of the pump but depending on the gas station, 1/4 to 1/2 gallon of gas that you paid for is still in the hose.

if that's what you are worried about, wouldn't you be getting the 1/4 to 1/2 gallon the previous guy left in THEIR hose? just sayin....

[Squillo]

Originally Posted by Splitter

Hand calculated is the way to go.

Onboard DIC adjusts your MPG every second the car is on an operating, if you idle for one minute, it will factor in one full minute of 0 MPGs

That's because when you're idling you're still burning fuel. How does hand calculating change this?

[Baba]

I used to maintain a hand calculated log but I've been using this website for the past 7 years for all of our vehicles. The link below is for my Camaro.

Data entered for each fill up: Total miles (odo), miles driven, cost per gallon, total gallons, onboard computer est. mpg & avg. speed (mph). Easy to see the difference between actual mpg vs. onboard computer est. mpg. Works like a charm every time.

http://www.mpgtune.com/vehicleDetails.php?carId=5547