There’s been quite a bit of chatter lately about the government’s latest Cash for Clunkers bill that would allow up to a $4500 car voucher to encourage drivers to upgrade to more Earth-friendly hybrids or super fuel-efficient cars.

But before you go running to your nearest dealership to snap up the latest Toyota Prius or Honda Insight, there are some critical criteria that must be met.

1. Your car must be 5 or more years old (2004 or prior)
2. Must be drivable AND currently registered for at least 120 days
3. For SUVs, the new or used vehicle selected MUST have an MPG at least 25% better than the government mandated 21.3 MPG.
4. For cars, the new or used vehicle MUST have an MPG of at least 25% better than the government mandated 27.5 MPG

So if you’re willing to do a little research and math, you can convert your evil SUV into a smiley hybrid. However, if you’re in charge of shuttling a soccer team around or require a large truck or SUV, you may not be able to benefit from the bill.

There are just four SUVs to choose from based on the numbers above, and while the car selection is larger, their size is not. Hybrid cars tend to be smaller, making it tough to downgrade from a minivan or large sedan.