I wish people would do the math for how much they will actually save going to a higher mpg car.

In this case, 100 mile commute. 500 miles a week, 52 weeks a year = 26000 miles.

I am going to give you the benefit of driving another 9000 on the weekends. So 35000 miles a year. That is a lot of miles, so cut in half the numbers below for normal driving.

Current - 27 mpg = 1296 gallons a year at 3.80 per gallon means $4924 a year on gas.

The highest mpg diesel is 42mpg, but you will get more like

34 or so average. Lets do both.

42 mpg = 833 gallons at $4 a gallon means $3333 a year

34 mpg = 1029 gallons at $4 a gallon means $4116 a year

Lets throw in a good Hybrid.

50 mpg = 700 gallons at $ 3.8 a gallon means $ 2660

So now you are paying $4924, the best you can do is $2660. Saving you $2200 a year. Your diesel will save you between $800 and $1500 a year. Is it worth buying a car just to get better gas mileage? The increase of insurance and taxes will eat in to that savings also. If you need a new car, any new non sport cars get over 30mpg. The difference between 27 and 50 isn't really that significant to justify driving a ****ing PRIUS.