Does air ever get trapped in the gas tank?
 

When filling up, can air get trapped in the gas tank making it seem like the tank is filled with gas when it isn't? I have been having too many weird tanks to come up with any other explanation. Any ideas?

Depending on the shape of the tank, and the vehicle's position while fueling, I guess getting an air bubble trapped in the tank is possible. I don't see it happening in a car that is level during fueling.

Do you use the same station and pump all the time? Pumps can vary in their sensitivity to auto shut off.

Buy yourself a 5 gallon gas can and buy gas from the station 5 gallons at a time. I have seen crooked gas station articles and how the pump can be set up to pump 5 gallons accurately but other amounts incorrectly. This also allows you to top off your tank wherever you like so that you can get all the air out.

Sometimes I wonder about how much gas I really put in the car. Sure some pumps are more sensitive than others, but the thing is, you can hear the gas filling the fill neck and approaching spill over. Yet, tanks where the sound was rising and it was approaching spill over (or did spill over) have fuel numbers that don't reflect a full tank. It's very strange.

You know it's only a 10 gallon tank on the cx vx. I usually once it clicks off on the first stop slowest fill speed. I then squeeze and fill as slowly as possible till it clicks off again. Then no more. With the cold temps your mpg is probably down quite a bit. I do like the 5 gallon idea. Or a 2 gallon just to keep the stations in check. I almost always use the same pump at the same station. Sometimes I shake the car a little to try to burp it. But we are talking about very little space here.

Why do you think those numbers didn't reflect a full tank? Calculated from long-term average MPG?

I wonder if that happened just because of excess turbulence frothing up the gas. Ever noticed that when you hear it rising and stop, it settles down like the head on a light beer and then you have room in your car's esophagus again? If you're patient you can top it off right to the rim like a fountain drink at a fast food joint. Just don't drink the gasoline.

My xb does that for another whole gallon and once it overflows it starts expanding and keeps coming out! Of course there is a rubber insert in the gas door that makes all the gas leak onto the outside of the fender and tire, you would think they would catch it, something I've been thinking of doing to it for a while now. Like a simple lip just inside the door to catch it and let it evaporate later or put a drain hole in it. I don't think the air bubble stay trapped in the gasoline in the tank for very long but it does foam up a little bit which is why it's recommended that you fill it more slowly. My tank is only 11.9 gallons and you really have to work the pump to get the last half gallon in there.

The problem when filling a tank that full is that you can damage your charcoal canister. Part of the evap system. So it's best to try to slowly fill the tank and when it clicks off be done. If you always use the same pump and the slowest auto setting on the pump it should be about the same. Hey if we really want to split hairs in the cold morning you could get another half ounce or so cold = dense.

Yeah, I have heard of about that problem, Rusty.

A. Read my gas logs for December 20 and December 24. Although just about all my best tanks have been driving down to NYC and back, whether it's taking route 22 south or I-91 south. So that's probably the mystery solved. The hills are much less going to NYC on those routes as compared to the hills East West on I-90 (Mass Pike). Still, though, the second half of the trip from NYC is all up hill and then driving into the hills of New Hampshire weren't doing any favors for my tank. So it's still pretty mysterious to me.

B. That could be all it is. I'm probably not patient enough to wait for the fizz to settle as it were.

The slowest auto setting will fill your tank higher. Filling it at full speed causes the earliest shutoff. That's why you can keep filling at low speed after the high speed shutoff is too aggressive...not that you should, though. As long as you're not topping off after auto-shutoff, low speed or high speed doesn't matter, you won't hurt your canister. (I am a reformed top-offer, I used to CLICK! CLICK! pssshhhhhCLICK! until there was no froth and it was spilling out...)