There was a story on the new the other night about a car here in Maryland. They had a locking gas cap on their car. So the thieves punched a hole in their gas line to get at the gasoline. So instead of being out the cost of a tank of gasoline, they had a $1,000 repair bill for their automobile.
Oh my!

An End to Gas Guzzling
Starting right after the 1973 oil crisis, most car manufacturers designed the filler necks of their new cars with a baffle a few inches down to prevent people from siphoning fuel. And just as well—the traditional method of starting a siphon by sucking on the end of the hose is a good way to aspirate gasoline and give yourself a case of chemical pneumonia.
You can buy a locking gas cap from an auto parts store, but if you have a newer vehicle it's probably not necessary. DH has a 1988 Chevy S-10 truck. He had his gas stolen two or three times while will lived on a city street, so we bought the locking cap for about $18 to prevent further siphons. We have a 2003 Pontiac Grand Am too, and the thieves didn't target that car.
DH is a mechanic and says newer vehicles are designed to keep the gas inside the tank, which prevents pulling the gas back out in traditional manners, like siphoning with a hose. So you may not need to spend the $20 or so for a locking cap.
That's what my son (former mechanic for GM) told me when I wanted a locking cap for my 2006 Malibu--that it wasn't needed. Now the vans at our park n ride have had gas siphioned out of them and they thieves did go in from under the vehicles. So that must be why (these are 2004 and newer vans) That is why we now leave our van almost empty over the weekend, adjust our time to leave ten minutes early Monday morning and gas up then for the week. If we stop midweek we try to put just enough in to get us through the week.
my grandmother's gas was siphoned out of her 2002 ford windstar a couple of times, so DH bought her a locking gas cap at advance auto parts. didn't stop the thieves-they just crawled under her front porch and stole the little 2 gallon gas jug she keeps filled to run her riding lawnmower instead. now when DH mows her yard, he takes his own gas jug and buys just enough gas to do the job, so they won't get much, if anything, when they try to siphon the lawnmower. IMHO, you have to be a real low-life to steal 2 lousy gallons of gas from an elderly widow.
We've had several night time incidents of gas stealing in our neighborhood over the last month. (We're sure it's someone in the neighborhood.) I too was going to get a locking gas cap, but was told not to bother - I have a newer Dodge Dakota. My DH has an older Jeep Cherokee and aready has a locking cap. NOW I'm worried about someone going in from underneath my truck, or ripping up the side of my DH's Jeep. We've also not been leaving them full.
Maybe if we leave a $50 taped to the side, they would just take it and go away? (Like that's gonna happen.) Or maybe a sign that says, "No Gas Left in Tank Overnight".
DisFlan