Anywhere not in your pocket. Where it'll get mixed in with the others and become just a coin. Which it is.

Here’s what everyone has been waiting for: A soon-to-be-ending saga of the gas tank update! Subtitled: How a gas station owner is full of it, and how Doug eats some crow. Just a little bit.

Since the last update, I’ve heard from a few people concerning gas tank size. Vince called me to say that he talked to his father-in-law about the story of gas tank sizes varying up to 2 gallons per car. His FiL is a former engineer for GM (to what capacity, I can’t remember), but he worked on the assembly lines, and could say with complete veracity that this guy was full of crap. While tanks could vary a little bit, it’s highly unlikely that a tank on one model would be off more than .1 gallons from a tank on another model. Reasonable, of course – the tanks are pre-built and assembled into the car. Anything off too much, not sure how it would fit. So we know the guy at Sudbury Mobil was talking out of his ass about that. But was he really cheating me on gas?

I assumed he must be, as I found out (as I stated before) that my tank is 13.2 gallons. To recap, this is what happened with my fill up at Sudbury Mobil on 6/27: (And remember, I normally only drive around 420 miles on a tank of gas, which brings me approx 40 MPG, and that’s when the tank is just below ¼ full.)

Gallons pumped in: 13.817 gallons
Miles driven on that tank: 428.8 miles
Calculated MPG, based on the fill up: 31.03
Car calculated MPG: 40.8

But here’s where it gets interesting. After that fill up, as I mentioned, the gas gauge didn’t go down. I mentioned this to Vince, and he said the electronic gauge is probably busted. But that would be kind of a big coincidence, right after this suspect gas fill up, that the gauge wouldn’t work. I drove over 170 miles on this tank before it finally started to inch its way down. On Friday I had gone over 400 miles, but my tank read just about ½ full. The question I posed to Sage, as we were about to drive up to NH for a weekend getaway, is whether I should trust my miles, or trust the gauge. Fill it up, she replied, so I did. That fill up (not at Sudbury Mobil) gave me the following data:

Gallons pumped in: 7.738 gallons
Miles driven on that tank: 405.6 miles
Calculated MPG, based on the fill up: 52.42
Car calculated MPG: 39.7

Well, isn’t this interesting. I filled up the tank until it couldn’t take any more, but it only took 7.738 gallons (a little less than half, based on the 13.2 capacity I was told I have). Both of these fill ups created wildly different MPGs as well. So then I thought, what would it look like if I combined the two fill ups?

Gallons pumped in: 21.56 gallons
Miles driven on 2 tanks: 834.4 miles
Calculated MPG, based on these 2 fill-ups: 38.71
Car calculated MPG, weighted average: 40.27

A-ha! So if I combine those last 2 fill-ups, I get a reasonable MPG, which is within 5% of what my car calculated. So how could this be? I think about it, and I can only come up with 3 thoughts:

1) The pump at Sudbury Mobil WAS off, telling me I was putting in more than I did, and then the other place I went to was off in the OTHER direction, telling me I put in less than I did. This is obviously pretty unlikely, and wouldn’t explain my gas gauge being so off.

2) My gas gauge is busted. This would not explain why my last fill-up was correct, according to the gauge; it said I was on half-tank, and I only filled up a half tank.

3) My gas tank is actually much larger than the 13.2 that Honda tells me it is. This is the most likely scenario; BUT how could it be that I’ve gone the same miles on a tank of gas, gotten the same less than ¼ full reading on my gauge, and never put in more than 12 gallons before?

Which leads me to a theory that I just came up with: My car, and maybe all cars, has some sort of over-flow or auxiliary tank. This over-flow only gets used when somehow the gas pump doesn’t shut off when it should, and it’s there to make sure that gas doesn’t go flying out of the car. It’s rare for this overflow to be used at all – maybe it’s only with certain shaped pump nozzles -- but when it is utilized, you obviously are putting more gas in the car than you normally would. This would explain the entire situation: When I filled up the first time, which should have only been about 11.5-12.0 gallons, after I put that much in the over-flow kicked in to “absorb” the other 2 gallons or so of fuel. The gas gauge is based in the main tank, and since the main tank stayed full while I used the overflow tank for the first 2-3 gallons, my gauge never moved.

I have no idea if this theory is correct; I guess I’ll call the Car Talk guys again, maybe they can answer this puzzler. Until then...the mystery continues.