Now that the House has approved rushing an additional $2 billion to prop up the financially strapped for cash for clunkers purchase program, owners of the 1991 Dodge Caravan might be thinking of trading in their older vehicle for something newer.

You might be in for a surprise when they get to the dealer.

Under the terms of the governments rebate program, a ’91 Dodge Caravan with a 3.0-liter V6 engine and three-speed automatic transmission, rated at 18 mpg, could qualify for a rebate. But the very similar ’91 Caravan with optional four-speed (TF604) transmission, rated at 19 mpg, does not.

That decision you made back in 1996 to pay $350 to get the used automatic transmission is going to cost you a few thousand dollars today. There are lots of examples like this in the minivan class, and I imagine it’s going to cause a lot of confusion when people turn in their cars.

Officially titled the Cash Allowance Rebate System, or CARS, the clunkers rebate program was launched just a week ago and lets buyers turn in an old vehicle and receive a voucher for $3,500 to $4,500 toward a new, more fuel efficient car. Cars and trucks must be 1984 models or newer and rated at 18 mpg or less to be eligible.

A government Web site listing thousands of models is the official spot to check whether a vehicle is eligible. (To check your vehicle, click here.)

The program is designed to revive weak U.S. car sales and get some old, gas-guzzling vehicles off the roads. It has proved so popular that it burned through its entire $1 billion budget in less than a week as new car buyers flocked to dealerships.

If the Senate ratifies the House measure and injects more money in the program, drivers may be surprised at what they find when they check on what they consider to be their old clunker.

The way I see it is an awful lot of people out there think that simply moving into a car that gets four MPG better than the one they trade in is going to be easy, but not that many cars qualify because 18 miles per gallon is a fairly low fuel economy.

You’re looking at a fairly old Ford Explorer with a V8 engine, a Chevy Tahoe or a Trailblazer, vehicles like that, but they need to be fairly old and meet all requirements to qualify.

Take, for example, the Buick Park Avenue, which was in production from 1991 to 1996. The car was big, luxurious and is seen by many as the quintessential American gas guzzler, but in reality the vehicle just misses the clunker programs with an MPG of 19 mpg. FYI: Mine, a 1995 Buick Road master wagon never gets less than 20 MPG on the highway.

It’s the same issue with many old American muscle cars. Often models with V8 engines qualify, while those with four-cylinder engines do not. A case in point is the Ford Mustang. Its 1990-1993 models, in particular.

It’s almost as if car shoppers are being punished for the decision they made when they bought their cars.

What appears to be good about this situation is there will be an overload of used transmissions and other recyclable parts for your clunker, especially if you can’t afford a new car or don’t qualify for credit. Good used transmissions for sale are in abundance now, make your best deal with GotTransmissions.com