If filling up your gas guzzler is eating up half your paycheck, you likely have thought about ditching it for a more fuel efficient vehicle.

Imagine, for instance, the savings if you traded in a Hummer for a Prius!

Well, Edmunds.com, an online provider of auto information, has come up with a reality check. Its online calculator allows you to plug in the make, model and year of your car and what you want to trade it in for. Then it figures if there's a savings or not.

"Car buyers think they can save money by dumping their gas-guzzler and purchasing a gas-sipper," says Edmund's Philip Reed in a statement. "But when you look at the big picture -- total cash out of pocket -- a very different situation is often revealed."

Edmunds uses the example of ditching a 4-year-old GMC Yukon worth $13,483 for a spanking new Toyota Prius worth $21,647. That's an $8,164 difference. If the consumer drives 1,200 miles per month and gas costs $4.07 per gallon, the monthly fuel savings will be $201.34.

Sounds good, but is it? It will take 41 months - more than 3 years - to pay back the extra cost of the Prius over the trade-in value of the Yukon, Edmunds reports.

I plugged in the information on a 2005 Hummer vs. 2008 Prius. As it turns out, there's no cost savings.

Play around with the site yourself. Let us know if you think it useful or not.