Well, I hope you didn't make a mess on yourself! Don't know what you'd
call it, but I certainly despise it. Every time you pump gas, it
automatically adjust the volume reading to what it would be if the
temperature of the fuel was 15 degrees centigrade. As a result, there is
no way of knowing exactly what volume of fuel you have put in your tank.
Mind you, I suspect that the temperature of underground storage tanks
does not vary a great deal from one day to another unless it has been
recently topped up from a delivery tanker that has been exposed to
temperature extremes while on the road.
I'm amazed at all the people quoting fuel consumption to the nearest 0.01
mpg. What are these figures and how are they obtained? I presume they are
with an mpg gauge fitted to the car, rather than from measuring the amount
of fuel required to brim-fill the tank from when it was previously
brim-filled, and the distance travelled on that tankful.
So are they instantaneous readings in identical conditions (eg 70 mph on the
same level route at 20 deg C with no cross/head/tail wind). Or are they
averages over the same journey.
Measuring my consumption by the fill-tank-and-note-distance method, over
about 550-650 miles, I get results which can vary by +/- 3 mpg depending on
variables such as route and mix of urban/rural/motorway driving and on
different cut-off points of different pumps.
The results are for a Peugeot 306 HDi (turbo-diesel) with manual
transmission, using mainly "supermarket brand" (Sainsbury's, Tesco, Asda)
Ultra Low Sulphur Diesel (ULSD), with occasional use of branded fuel (eg
Shell, Esso, BP, Total).
It's reassuring the see that over nearly 130,000 miles, there hasn't been a
noticeable reduction in the mpg as the car has got older: the only
significant change was that the consumption was worse when the car was new
until the engine got thoroughly run-in.
The figures are miles per UK gallon, so multiply them by 4/5 to convert them
to US gallons.
The blue points/lines are the actual readings for successive tankfuls. The
red line is a moving average over three values. The pink line (at just under
50) is the overall average since new: total distance / total fuel.
The readings of around 58-59 are for long motorway journeys at 70 and then
journeys at around 50m, with very few short journeys or around-town driving,
when I was on holiday. The rest are a random mix of journeys at 30-50 for
about 15 miles, with a few longer ones thrown in. I don't have a need to do
much travelling in heavy traffic, so there isn't the frequent
Interesting that the results were consistently bad (41-44 mpg) when the car
was new until the car was about 7 months old and had done about 8000 miles.
That's a long time for the engine bearings to slacken off!
It's just the results of simple division. Do what you will the
The method I've used is at the start of a trip the first fill is to
the brim, and write down the odo reading. Every other fill is to an
even gallon on the pump until I get home when I fill to the brim
again, and jot down the final odo reading. On the trip I fill to the
even gallon, 10, 11, 14, whatever, and jot it down. I don't worry
about a couple hundredths if I go over an even gallon, and don't
bother with the odo again until I'm back home.
So at the end I've done 2533 mile and used 81 gallons.
Doing a division using Windows calculator I get
Of course I would say if asked my trip mileage, "A tad over 31."
If I do the same trip next year in the same car it should be close to
the same, so I pretty much know my gas costs.
I would trust this method more than any onboard car fuel metering
device, but know it's not perfect. Weather, driving conditions, tire
size/inflation, etc, etc.
Frankly, I only do this once after I get a car to establish a
reasonably close expectation of mileage.
After that I just gas when needed, and hardly ever think about it.
Those who want to measure mileage could do the same on a yearly
basis and be pretty confident about their overall mileage.
I don't know how the car consumption meters operate, but it would be
interesting to see the variance over a year between what they say and
what was actually pumped into the car according to the gas pump
My impression of the calibration of the meters on the gas station
pumps is that it's good. I'm pretty sure there are plenty of
volunteer "consumer watchdogs" with exact gallon cans checking them
out, and itching to go after the oil companies if they are
But maybe they are undermetering a bit to keep them off their backs.
It could cost them quite a bit if they were caught shorting the
Two tankfuls does not make a very good trial. In order to compute
variance of a measurement you should take about ten, and compare the
resulting variance to the differences between milage with different
gases. A difference less than the variance is meaningless.
Fortunately, many calculators these days will compute variance as well
as average, so the statistics are a lot easier these days.
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