My Chevy Avalanche has a 33 gallon tank. I can use regular so right now the average fill up is running me just at $100 if I fill up as soon as the gas light comes on. I go through about a tank a week on a bad week and a tank every 2 weeks on a good week so Lets say I spend about $100 every week.

Given that my truck is paid off I dont have any expenses there nor do I have to worry about any kind of equity investment so with that said here is my logic:

I pay $400 a month in gas.
If I were to trade in my truck I *might* get $10,000
If I were to buy a Prius they run about $28-$30,000
That would leave me $20K to finance
Over the course of 4 years that would make my payment roughly $400 a month which is equal to what I am paying for gas now.
I would then have to put gas in the new car which would run me about $50 a week
Given those numbers I would be spending an additional $50 a week above what I am spending now.

Remove the maintenance aspects from these equations and I am losing money by trading in my truck therefore I am not trading in my vehicle any time soon.

If gas prices get so bad that it becomes prohibitively expensive to fill my truck up, Ill ride my motorcycle (which, coincidentally, is also paid off and gets about 35mpg)

Make sure you do the math first and make sure you arent better off just keeping the car and paying for the gas.