Sudden Mileage Increase: Confused

[Doogals]

New member, been tracking the fuel consumption of my 2007 Pontiac G6 GT since May of this year.

Over the past few months I was averaging 11.2L/100 km (21 MPG), but for some reason my last tank returned 9.0L/100 km (26MPG). Other than one brief 10km (6 mile) highway trip on this tank, the rest of the driving was identical to all of the previous fill-ups before it. I'm confused why the sudden 20% increase in fuel economy?

This one has left me stumped, see data below. Any ideas?

31-May-11 11.6
14-Jun-11 11.3
27-Jun-11 11.1
12-Jul-11 10.8
29-Jul-11 11.4
15-Aug-11 9.0

[Doogals]

(31-May) 11.6 L/100 km
(14-Jun) 11.3 L/100 km
(27-Jun) 11.1 L/100 km
(12-Jul) 10.8 L/100 km
(29-Jul) 11.4 L/100 km

(15-Aug) 9.0L /100 km

[theholycow]

Assuming everything is consistent, I would expect that to be the pump shutting off early before your tank is full. If so, it should become evident after more tanks.

[Doogals]

Bingo.

Just checked, my average fill-up previously was 55.5L (14.6 gallons) and my last fill-up was 44.5L (11.7). So a difference of 11L (3 gallons), that explains it.

My fault for going to a full serve gas station and not paying attention to the total. Too bad though, as this puts one hell of a drop in my fuel consumption graph...

Thanks for the help.

[GasSavers_BEEF]

when you fill up next time your consumption will be probably 20% (or there abouts) lower so the two will probably average out.

there are several instances when I have done this but the difference hasn't been that much. we don't have full service stations around here (that i am aware of)

[Doogals]

The difference checks out I believe. I was confused why my fuel economy suddenly improved by 20%, but after discovering I only received 80% of a full tank it makes sense. The number of litres to refill the tank, divided by the number of km driven since the last fill-up, provides the fuel consumption rate of L/km. However because my tank was not completely filled, it appears I drove all of those KM on less fuel.

This also does a good job of explaining it:

http://www.fuelly.com/faq/3/fill-the-tank

[kamesama980]

in my own excel sheets for the cars, the FE tables include a column to average with the next and previous tanks. It shows they tend to even out.

[GasSavers_BEEF]

I've heard that to get a good sampling, you need to go through at least 3 tanks of fuel. in my yaris, that is almost 3 months.