How much does gasoline expand with temperature?

[Dakota]

There was a thread earlier this week about a member who has a problem with fuel dripping out through his gas cap while the car was in storage.   That got the geek in me to look into how much gas volume actually changes with temperature.   It turns out there is a readily available figure from the U.S. General Accounting Office (GAO):   1% of every 15 degree Fahrenheit temperature increase.

Soooo..... let's say it's a late Fall day and you're getting ready (reluctantly) to put your beloved eBody away for the winter.   Some assumptions:

1.  The gas in the underground storage at your local gas station has cooled to 40 degrees.   

2.  You end up with 20.0 gallons of gas in your tank at the exact moment the pump automatically shuts off.

3.  We'll conveniently ignore how much gas you burned driving home any losses due to evaporation while the car's in storage. 

Fast forward a couple of months of searching for parts, reading jimmynick's colorful stories, making lists and checking them twice, etc etc, and you are suddenly blessed with a string of nice warm 70 degree days.   How much gas do you have in your car's tank now? Since the temperature is up 30 degrees since you filled the tank, you now have 2% more which translates to 20.4 gallons or nearly 2 more quarts that what you put in the tank in the Fall.   That's great, except if the gas has no where else to go but out the gas cap.

I saw some posts where the author argued that you should fill your car up in the morning because the temperature is cooler compared to the middle afternoon.   The problem with this thinking (I think) is that what matters is the temperature of the gas in the station's underground tank when you pump it, not the air temperature.   The temperature of the gas in the underground tank isn't going to vary very much over the course of the day, but, on the flip side, every little bit helps.

The morale of the story:  leave some room for expansion in your tank if your car is getting tucked away cold and will "wake up" with warmer weather.  The amount of room depends on your local weather.

That's probably more than enough, so I'll stop here.

[70 Challenger Lover]

That's actually very interesting. I appreciate analytical thinking like this. I've been having an annoying fuel leak out the vent lines on my 69 Coronet and while everyone has advice on the problem, I've been thinking about the fuel expansion issue myself coupled with poor design.

My problem only develops when I fill the car up on a warm day. By the time I've driven home, the fuel in the tank has to be nice and warm sitting under a warm trunk space and nestled in between two hot exhaust pipes. I figure the underground tanks are about 60 degrees but I'd bet the fuel in the tank hits 90 pretty easy at some point soon after. That's a couple quarts expansion by your calculations.

[Brads70]

I guess that is what the stock set up in the trunk if for? If it's missing it might cause the issues mentioned.
Also I'd guess Cuda's suffer potentially  more from this due to where the filler neck is as compared to Challengers? 

These cars were never meant to have a full tank for very long anyhow! If yours is full all the time ...." your doing it wrong"    Get out there and drive them!

[70 Challenger Lover]

I have all the correct vent lines. Everything is new. I believe expansion removes the remaining air at the top of the inside of the tank and starts pushing gas out the vents. I see tons of people complaining about the same issue on this setup.

[7212Mopar]

Gasoline is considered volatile compound, low vapor pressure. It will expand with temperature just like any material but convert to vapor faster if vent to atmosphere. The rate of vaporization increase as temperature rise.

[tparker]

hmmmm... this sounds fishy to me. Maybe it is correct but i think there are other factors at play. I've been filling up gas tanks for over 30 years and never had fuel leak out of the gas cap. I live in California were we get over 100 degree temperatures regularly. If this was a thing I would think you would see a lot gas spillage since google says there are 276 million cars.

EDIT:
after googling around, apparently this is a thing with older cars. Huh, never had an issue myself.

[jimynick]

"after googling around, apparently this is a thing with older cars. Huh, never had an issue myself."  and given the price of gas these days, you shan't have too much of it in future, either. LOL How about, just not filling your tank to the tippy top? OR, if you do, take the car out on the road and lower the load a wee bit while simultaneously putting a big grin on your mug! 

[captcolour]

I'd have to agree with 7212Mopar, it isn't about liquid expansion, but about evaporation rate.  No data to go by, but my guess is that since our old cars have metal gas caps, the evaporating gas condenses due to the change in temperature hitting a colder metal gas cap.  What is dripping is condensate.  A quick test would be to put a plastic gas cap in and see if the problem goes away.  If you can find a plastic one that is.  Just my 2 cents.

[Dakota]

I agree that the "real world" issue in the post about damaged paint could easily be more about condensate dripping. As I stated in the assumptions above, I was ignoring evaporation (just to keep things simple), so the volume expansion calculation in my original post only applies to a completely closed system (no vent line and a sealed gas gap).

[YellowThumper]

"after googling around, apparently this is a thing with older cars. Huh, never had an issue myself."  and given the price of gas these days, you shan't have too much of it in future, either. LOL How about, just not filling your tank to the tippy top? OR, if you do, take the car out on the road and lower the load a wee bit while simultaneously putting a big grin on your mug! 

I also have a 68 Mustang with filler in the back similar to a Cuda.
Also in CA. And yes, I have that issue if it is filled and then not driven. Fuel expands and leaks out.