we all know the MPG display is simply inaccurate.

ive only used it to measure entire tanks of fuel.

now i want to get an idea of my mileage based purely on my daily commute.
so i'm going to reset the counter every day, and get a week's worth of MPG readings for my daily roundtrip to work.

here's my question (and i know the answer is speculative).... is there any reason to believe the MPG indicator will be more or less accurate when measuring a 30 mile trip, as compared to a 300 mile trip?

on my first 7 tanks of fuel, the computer has been off by
+3.7 mi.
+4.0 mi.
+4.0 mi.
+3.9 mi.
+2.8 mi.
+3.5 mi.
+3.9 mi.

 

It looks like the MPG calculator is consistently off by a certain %, in which case it will not matter if you take 30mi segments and compile the data later.

 

ive heard people are getting a variety percentages. and i was under the impression the more actual mpg you get, the greater the discrepancy the computer will generate.

 

Are you looking for mileage or an mpg calculation of your commute to work? For the former, you could simply use google maps. For the latter, why not drive it until the tank is empty, note the mileage, and calculate against the next fillup?

 

i want to know how many miles i'm getting per gallon on my commute.
nothing else.

 

I've heard it's always about ten percent off. That seems to hold true on the three tanks of gas I have had.

 

Since I do not know exactly how the MPG indicator is programmed, let me put forth this bone to chew on, and this would account for the differences people are seeing...

When you calculate your MPG for your commute by hand, you are using TWO factors to get the result. The miles driven and the number of gallons used. But there are really THREE factors that are a result of that MPG, because included in the miles driven is another factor...the amount of time spent idling which uses gas, but does NOT add miles. OK?

What I am THINKING is that the MPG indicator is ONLY taking into account the miles when the car is moving, which would make it look like no gas was used while idling. So you would have a much better MPG.

I'm thinking that it's tied into the trip odometer, the regular odometer.

So that long trips that are non-stop should be relatively accurate as far as that MPG indicator. But in stop and go traffic, it will get wacky and very much off. Because of all the gas being burned while at a stop and not being accounted for in the MPG...

It is MILES per gallon, and you aren't getting ANY miles when idling. See what I'm saying? So it's not deducting that gas usage while idling.

The best way to test out this hypothesis is to fill up close to the freeway, get on the freeway during non-commute times and take a long boring drive to a gas station 60 miles away.

The point of driving on the freeway (preferably a flat one) is to never idle, to keep a very steady speed, without a lot of accelerating and braking.

And, obviously, when you are at a stop with the car running, you are getting zero miles. If you reset the trip odometer, and idle away an entire tank of gas, you'd get zero miles per gallon, right?

Now, if you were on the freeway, for that long drive, and you got a great 45 MPG you'd be happy. Then you pull over and put the car in idle, and burn up the rest of the fuel. It will still show 45 MPG.

But your manual calculation would be very different. Because you still used that gas and it's calculated into the amount of gallons you needed to fill the car up again. Right?

Does this make sense?

 

the concept sounds good. but, in fact, the computer does take idling into account.... because as one sits at a red light, one can see the MPG number actually tick downward on the display.

btw, my commute is 28 miles round trip, and i have to go through 42 traffic lights round trip. the MPG indicator for the last 2 days of my commute reads 36.3 mpg. so i figure at worst i'm actually getting 32.3 mpg.... which isnt that bad with the amount of traffic and traffic lights i encounter.

 

Ahh.

It couldn't go less than zero, though, could it? Like if you reset it now, and just sat idling, it wouldn't make a negative MPG, would it?

 

If you get a scangaugeII it is much more accurate than an onboard one, just plugs into OBDII port, 125 bux

 

Scanguage is not more accurate. It uses the same data as Fit's BSM. But one can adjust it for error correction.

 

Only one way to find out. Are you volunteering?

 

It wouldn't go less than 0. Once after I filled up and reset the trip odometer, I happened to let the car idle for a bit while I ran into the convenience store for a drink. Stayed at 0.

 

If one thinks about it, zero mile traveled divided by fuel used will always be zero.

 

What causes the mpg indicator to be so inaccurate? If we are forced to check our mpg the old fashioned way, what in the hell is the point of them installing this in the first place? Can it be recalibrated or given a software update?

 

scangaugeII you can recalibrate for corrections

 

I'd like an answer for this as well. I realize the Scangauge can be calibrated, but I don't particularly want to spend $125 for a function I should already have.