What can cause a car's mileage to drop?

My car is a 2007 Toyota Camry. Last year, a drunk driver slammed into the back of it while it was parked on the street. Geico took really good care of it, but on the night of the accident, I noticed some fluid on the ground. I mentioned this to the insurance agent, and suggested it might be a gas leak. The agent said that if it was damage to the gas tank, she could probably smell it.

Before the accident, I was getting around 20-25 miles per gallon, depending on freeway or city driving. After the accident, I am now getting 14-15 mpg.

I have already talked to a repair shop about checking for a gas leak, but they said it will cost $100 for the test alone (they use a smoke machine and dye to detect leaks.) Before I do this, I was wondering: is there anything else that could cause this drop in mpg?

I noticed another thing: when I leave my car parked in a cool place, I lose less gas. When I leave it parked in the sun, I often lose ~1 gallon per 3 days without driving the car. When I leave it parked in the shade, there’s little or no evaporation.

I would suspect a leak. I had an old car that had some larger pinholes develop (because of rust) in the top of the tank. I never leaked fluid that could be noticed but the mileage did go to heck much as you report.

What kind of damage are we talking about (technical details or repair cost)?

A common cause of mileage drop after a major accident is that the alignment isn’t right. This can be difficult to do after repairing major frame damage. If I were you, I’d call the shop that did the repair and tell them about the drop in mileage. If they’re a good shop (big “if” sometimes) they should be happy to re-check the alignment and/or check for any other possible issues with their work (after all, they might find something else to bill your insurance company for!)

I think a gas leak is probably unlikely because, for one, you probably would smell a gas leak that is severe enough to affect your mileage and I believe the evaporative emissions control systems would notice that the gas tank is not fully sealed and would trigger a check engine light.

Good idea about the alignment. Yes, there was frame damage, but it was isolated to the rear.

However, I seem to be losing gas when I’m not driving. When I do drive, I appear to get 20+ mpg. When I start it in the morning, the gas needle is noticeably lower.

Yes, the repair shop said I should see a dashboard light, but my car automatically turns it on at 5000 miles for oil changes, so we can’t tell from that.

My suspicion is that the neck of the tank is damaged, which is why it’s not triggering the regular sensors. If it’s cracked, the gas is evaporating, if it’s bent, maybe the gas cap isn’t able to maintain proper pressure when the gas is heated.

The light that comes on for oil changes is different than the one that comes on for problems with the emissions equipment. However, I’m not thoroughly familiar with how the newer EVAP systems work and so it could be possible that since this system only really tests to see if the vacant space in the tank is leaking, maybe it wouldn’t notice a leak that’s actually leaking liquid gas. Also the neck of the tank is covered by these sensors-- that’s why you get a check engine light if you drive around with the gas cap loose.

Also, while gasoline does evaporate in small quantities, significant quantities of it won’t evaporate out of your gas tank. Old cars didn’t have sealed gas tanks at all, and they didn’t have any problems with gas disappearing in any great quantity.

To do some quick back of the envelope calculations, say you’ve got a 16 gallon tank and you usually get 400 miles out of it @25 mpg. But now you’re only getting 240 out of it giving you an indicated 15 mpg. If your car was still getting 25 mpg, you’d have to be leaking about 6 and a half gallons in the time it took you to drive those 240 miles-- that’s a heckuva lot of gas. Considering that it only takes drops to make a noticeable gasoline smell, I think a simple leak is probably not your issue.

I suggest parking the car such that the gas tank is above an absorbent material like painter’s paper.
Check for gas in the morning.

Seriously, though, something here is fishy.
Could you try taking pictures of the gas gauge before and after you park the car?

On edit:
BTW, my bet is on frame damage.
Also, double-check tire pressure and your tire gauge if applicable.

So you have ruled out pilfering? You know, over there in the sunny spot, by that guy with the hose and the gas can.

Assuming the check engine light is operating on your car it almost assuredly isn’t a gas leak. A 2007 car should be check for a 0.5mm leak on a regular basis.
The alignment would have to be pretty damn bad to cause a 10 mpg drop. Does the car steer straight, and when you take your hands off the wheel on a level road does the car track straight? (not veer right or left)
The poor man’s frame check is to drive straight through a puddle and observe the wet tracks out of the puddle. There should be two the same width as the tire tread. Wider than that, or worse case 4 separate tracks indicate an alignment problem.
A bad O2 sensor could cause such a problem and might not trip a check engine light. Is the car running normally?