This time around, I thought I was doing fairly well considering my trip. On my latest tank, I had mastered a few new techniques, and in general practiced DWL better than usual among other things. My tank average was 53.3 on the fuel display, which is pretty good for me considering I have fairly short trips, always ending up a hill that costs me big time.

However, my fuel light came on a lot earlier than anticipated. When I filled up and compared the miles driven on my trip odometer to the gallons purchased, my mileage came to 42.8mpg! That's more than a 20% discrepancy, and highly unusual considering my very conservative driving technique.

The thing that I'm confused about though is that the sales receipt said I purchased 13.278 gallons... (I ran the pump until it clicked, didn't overfill) Considering the fuel warning light just came on, and there should have been about a half-gallon left at least, it later occurred to me the station's pump might be miscalibrated (I later remembered the tank is only about 13 gallons)

Has anyone else noticed anything like this before?

 

I have had situations where the pump malfunctioned somehow, and auto-clicked off after putting well over 12 gallons into my 11.8 gallon tank -- and I was nowhere near empty. So my mileage for that trip was very poor.

At my next fillup, lo and behold I got absolutely tremendous mileage.

My interpretation is that the pump and it's autostop mechanism malfunctioned. It was one of those pumps you get sometimes where the gas just barely trickles into the tank, very slowly. And it just keeps right on filling, with the autostop mechanism not getting triggered until the tank was overfull.

I've had this happen to me twice, and both times, the subsequent mileage at the next tank of gas (calculating by hand) was well beyond what I should have received.

 

These things have a way of working out in the long run. As coyote said really bad (or good) tanks are usually followed by a "correction" tank that goes the other way.

 

I've had the same happen before and it was obviously the gas station pump being severely miscalculated. Some would argue that the gas station wouldn't be 1 gallon off for 12 gallons of fuel but I think it is possible.

 

If you use the "first click, no more gas" method of filling up a tank (nothing wrong with that), you will have to live with the fact that the first click may come at different points in the fill process. I have had it click off at 5 gallons, when I *know* I need to have 7 more. I just tell the petroluem dispensing technican that the click was in error and they continue along.

Thus, the pump, the ambient temperature, the amount of gas left in the tank, and the price of coffe will influence when that pump will click off.

Therefore, remember, that time and data are your friends! As you accumulate your tanks and fillups, the MPG overall will level out and approach the actual LMPG for the car.

 

It is also more consistent if you use the slow fill rate rather than the fast one. Too fast causes gas to slosh upwards and cause erroneous click offs. I stoppped using the fast rate years ago when I had an incident of it clicking off when the tank was nowhere near full.

 

One of my tanks calculated to 81MPG last June.
How? I don't know. Gross pump error I guess?

I also belive that different pumps click off at different levels than others.
Perhaps it's rate of flow causing backsplash within the filler neck.

 

Since I fill up to neck (ALL THE WAY UP) I can attest to certain pumps being "kind" while others work like a binary switch - they are either off or firehosing gas out with no ability to trickle gas in. I note those gas stations, and don't go back to them. Otherwise it takes me 5 minutes to fill the car.
I'm sure the "click point" is also affected by the rate of delivery of the pump too.

 

Also, remember that it depends on the angle of the car. Most stations try to have flat surfaces to fill, but most have some slope. I have noticed a big difference when the nose of the car is lower than the back by up to a couple gallons, depending on the angle.

 

Sooooo, which way did the difference go? + or -?