While the country has watched gas prices work their way up past four bucks a gallon, some people have been breathing a small sigh of relief. 

Cars converted to run on used vegetable oil have gotten more popular over the last few years.

In today's Money Saver, NBC17 shows that if you've got a little elbow grease and a lot of veggie oil, it might not be a bad time to make the switch.

Gary Wilser and his son, Alan, say spending $500 and up to convert cars to run on veggie oil isn't for everyone, but so far it's working well for them.

"I'm saving maybe a couple hundred bucks a month," said 28-year-old Alan. "So that's a pretty big plus."

Over the last three years, the elder Wilser has converted more than a dozen cars, trucks and even a bus.

The family gets their fuel from restaurants for free, and sends it through several rounds of filtration before it's ready for the road.

"It's sort of a nasty little job," said Alan, looking at one of the filters with a wry smile. "We got some beans in there, it looks like. Maybe a carrot. Onion."

"It's a little messy," laughed Gary Wilser. "That's one of the drawbacks of this whole operation. It's not as easy as pulling up to a gas station and sticking a nozzle in the tank."

Cars running off vegetable oil have to have diesel engines; regular unleaded won't work.

"This has an 18-gallon aluminum tank that's part of a kit that's mounted in the trunk," said Gary, pointing to the trunk of his 1978 Mercedes. "You have to start the car with diesel, let it warm up to operating temperature. Once you're warmed up, you flip the switch one direction and you're running on vegetable oil."

Wilser says it's not really worthwhile for short trips, but on a long haul, he can save a bundle.

In fact, Alan and some friends took their converted bus 8,000 miles around the country last summer, and used less than 150 gallons of diesel.

He said that total would have been less, if not for some mechanical problems.

Gary Wilser said it's not just the fuel savings that he and his family like about the change.

"The engine runs smoother, quieter, it pollutes less," said Wilser. "The exhaust even smells good -- smells like something cooking."

For now, many restaurants are happy to give away their used oil, but Wilser says he can foresee a day when they'll realize there's an opportunity to make some money; and when that day comes, he'll be ready.

"Ride the wave while it lasts," he said. "When it quits working, I'll get into something else."

Of the 14 conversions Wilser has done, some have been for friends of his son, while most have been rotated in and out of the family's daily routine and sold one at a time; they still have three that they use.