Another gas mileage question

Factual Questions
1

Last month, I asked a question in GQ about filling up my gas tank. I have another one now!

Last night, when it was about 60-some degrees out, I stopped at a gas station and filled my tank all the way.

I then drove home 10.6 miles (according to Google) on the freeway.

Today, when it was 70-ish and sunny, I drove 18.6 miles on the freeway to somewhere and 18.6 miles on the freeway back.

Since I last filled up my tank, I had driven 47.8 miles almost exclusively on the freeway.

After driving those miles, I stopped at the gas station to fill up again (I had driven for work, so I needed to fill up on the company’s dime).

After driving those 47.8 miles, I only was able to put .876 gallons of gas into my tank before the automatic shutoff at BP stopped the pump. According to my math (which may be bad), that’s 54.56 mpg.

Obviously since I drive a 2004 Ford Escape (automatic) and nothing even close to a hybrid, that number is not the true highway MPG for my car. I do believe it’s advertised at 25 mpg.

So my scientific reasoning leads me to believe that BP’s pump shut itself off way before I was full. But that much? If I truly get 25 mpg then I would have needed about 1.9gal of gas and thus I got 1 gallon less than I “needed.”

I can see the number fudging a bit to accommodate for fumes but a whole gallon’s worth?

And, if this is pretty normal then is keeping track of your actual mpg sort of a losing proposition since you can “lose” a gallon here or there based on automatic shutoff?

Curious minds want to know!

2

Automatic nozzle shutoffs have a lot of variation. The only way to get an accurate MPG figure is to either
a) keep “clicking” the nozzle until it won’t stay on for even a moment (frowned upon, because it can lead to spills), or
b) keep a running average, which will compensate for partially-full tanks.

3

Best compensating mechanism is to always use the same pump, same station.
Either that, or just calculate total gallons & total miles by adding together 5 fill-ups.

4

A minor factor that may contribute to the problem is thermal expansion of the gas in your tank when the ambient temperature is greater than that of the gas as it comes from its underground tank.

But no doubt the big problem is that of trying to fill your tank to the same level each time. Very few cars make this easy to do. The inaccuracy in your calculated gas mileage will tend to be greater when the amount you’re pumping is small, as in the example above.

5

Of course, there is always the possibility that you were doing 32 mph all the way there and all the way back with a tailwind, while following a large vehicle very closely, in which case you might actually squeeze 50 mpg out of an Escape.

Interesting side note: if you Google “theoretical maximum fuel efficiency”, we’re the 13th hit:slight_smile: