DIYbanter - View Single Post - What is the realistic accuracy & precision of typical consumer MPG calculations (tripmeter miles/pump gallons)

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On Thu, 20 Jul 2017 10:57:06 -0400, Ralph Mowery
wrote:

In article , says...

wrote:
On Thu, 20 Jul 2017 12:08:33 -0000 (UTC), Mad Roger
wrote:

What is a realistic accuracy & precision of typical MPG measurements when
measured by the consumer using the typical method of dividing their
tripmeter miles by the gas-pump gallons during fillup?
Repeatabilty is terrible. Accuracy can be pretty good over multiple
tanks. Can be pretty good even on single tanks IF there is a way to
ensure the tank is always filled to EXACTLY the same point (like a
level in the fill - tube, with the vehicle parked at EXACTLY the same
place for each fill-up). Relying on the auto-shutoff of the pump can
cause variance of several liters per fillup.

Vehicle position or the auto shut off point won't make any difference.
You read the amount of fuel you pumped off the pump itself. The only
real issue is odometer accuracy. That can vary with tire size variations
and factory calibration.

If you just do it one time, you can not be sure you put in the same
ammount of fuel that was taken out.

If you keep a running total of the ammount of miles and fuel over
several tank fulls , the ammount of fuel will sort of average out.

Say you park so the back of the car is up hill and you fill the tank.
Go a number of miles and fill up again. This time the back of the car
is down hill. You may burn out 15 gallons, but only put back in 14
gallons. Ot it could be the other way around and you burn 14 gallons,
but only put back in 13 gallons. From tank to tank full there could be
a large variation. Over many tanks, the variation will average out to a
lessor error.
After say 10 tanks used you only have to contend with one or two errors
caused by the exact ammount of fuel put in the tank. Probably just the
last tank full would be where the error would come in. So instead of 1
gallon of error like the example above, you would have about .1 gallon
of error if the pumps are correct, which they should be.

If the first and last are identical, none of the others matter. The
difference of 1, or 10 liters spread across many tanks becomes , more
or less, just noise. On the short term, like 1 tank, it can be a
pretty large percentage of error.