After 61,000 miles on my 2003 Civic hybrid, I have averaged
54.9 mpg according to dash gauge, but
49.9 mpg according to my gas receipts.
When you folks tell your mpg,
which are you referring to ?

 

Hi Jerry Nicholson:

___Some guys liked to post only the fuel consumption screen data and we have debated this to the ends of the earth in the past but please just post your actuals (actual miles traveled/actual gallons purchased) as this is the actual FE you achieve, not what the digital gauge(s) display. On a similar note, the lmpg of my Insight matches exactly with the lmpg calculated by actual miles traveled/actual gallons purchased. I know Eric Becky re-uploaded his Insight’s entire tank over tank history because he was using the gauge displays but saw an ever growing discrepancy developing between the consumption gauges and his actuals IIRC?

___Would you also mind posting your tank over tank data as I want to add your accomplishments to the Hypermiler list. During this time of year, that thread is a little slow as you can imagine given the cold temperatures most of us are forced to drive in …

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___Waynegerdes@earthlink.net

 

This problem will remain until Jason makes a column for both dash and at the pump.

The delta would be an additional data point that would be interesting to trend.

 

Generally, I feel the calculation method is more accurate, although it's prone to errors of its own, but they tend to equal out over time. For example, if you "top up" tank 1, the performance for that tank will look worse than it should, because you've added more fuel, which you "used" in that tank. Then if you don't top up the next tank, that tank will look all the more impressive, because you didn't put in the same amount of fuel as you burned.

This method pretty much proved to me that to my dealer "a full tank of gas" means "just enough so that all the bars on the gas guage are filled."

 

So I wonder, why the MPG discrepancy? With an opportunity to precisely measure the real MPG, why would Honda or Toyota or Ford have their calculations be off?

I understand that for some of you, the dashboard values are correct. But for a number of folks, myself included, it seems like the dashboard values are "optimistic". My difference seems to be about 1 MPG (e.g. 45 dashboard is really 44 measured).

I would not *think* that the automobile manufacturers would be trying to pull a fast one on us. I *might* think that the oil companies would pull something like this, but then I don't think they run the gas station equipment, do they? I think that the pumps themselves are supposed to be correct -- they are probably required to by law -- but then, I have heard that meters in taxis have been "miscalibrated" to the driver's advantage, so...

Anyway: does anybody have any theories why the mileage discrepancy might exist? Maybe a technical reason -- e.g. gas blends, evaporation, or something?

 

The miles per gallon readings on the FCD on my 2004 manual tranny HCH varies widely from the traditional "miles divided by gallons" calculations. And it's not ALWAYS optimistic:

Here are my first 13 tanks:

FCD M/G variance

37.8 38.4 (0.6)
43.1 40.7 2.4
45.0 42.9 2.1
48.4 48.5 (0.1)
53.3 49.7 3.6
55.6 53.9 1.7
54.8 51.3 3.5
52.6 49.9 2.7
52.6 46.8 5.8
47.3 52.8 (5.5)
46.1 43.4 2.7
46.5 43.4 3.1
46.9 49.9 (3.0)

I am at a loss to explain the discrepancies also. It's only an AVERAGE of 1.43 miles per gallon difference between the FCD and the M/G, so it's not a MAJOR problem. But it is OFF.

 

After 2,456 miles my display (trip "B") shows 60.1 mpg and the database here (based on actual fillups and trip mileage) shows 58.4 mpg. This makes the display in my MT HCH 1.7 mpg or 2.9 percent off which makes mine very close to Lars-ss'es (OMG that is a lot of "s"es) as his is 3.0 percent off (1.43 / 47 * 100).

Actual fill data is right on the money in the long run but is useless for tank to tank data because of differences in fill level. The gauge display is not useful for most of us because they tend to be optomistic. Neither way is perfect. The best way is to use corrected display figures but that is a bit complicated and impossible in the short term.

 

My calculated mpg tends to run a bit lower than the dash.

I use the caluclated because I feel it is better to understate my mpg that to overstate it.

I figure if I am understating my mpg it takes a bit of ammunition away from someone who might otherwise say that my mpg is not "real".

If you look at my entries I enter in the calulated, but always identify the dash mpg in the "Other notes" area.

Example

Although I entered 74.9 in the database for my tank shared on November 23rd, I also wrote this in the Other Notes section:
11/20/04 fill up. 9.677 gallons. $1.969 per gallon. Dash: 75.2

 

your gallons in the tank can vary also.

example

VW diesel and gas use the same tank system. I did the vent removal on my diesel and gained over 2 gallons of holding capacity.

This extra space is for expansion of gas and the pressure is released during this expansion. When the gas cools down that causes a slight vaccum thus the sucking air in when you remove the cap. Since the gas tanks can hold more you could get some differences but should be minor. Also one or more hybrids use an expanding bladder which can greatly effect readings.

Diesel doesn't expand like gas so the vent mod is safe. I wouldn't recomend it for gassers.