Gas prices went up already, 30 cents from Wednesday to Saturday night at our Safeway.
If you are looking for just for MPG in your car then you can find cars that will net you 40+mpg. Now you added the fact that you want it to last 10+ years too that changes things a little. Do you know how many miles you plan on driving it per year?
Either way if you do end up looking at a diesel remember to factor in all the costs. The biggest one is the initial cost of the engine upgrade. In the new truck world you can count on a $6000 bump when going from gas to diesel. If I recall right when looking at the VW Passat a few years ago the upgrade ran about $2500, so you have to add that to your cost per mile. If you are planning to keep the car for 10 years then you should recoup that cost.
Also as Corbet mentioned the PM can be costlier. Engine holds more oil, fuel filter needs to be replaced more often, injectors can clog easier than gas ones. Also the parts tend to be more expensive.
Not trying to talk you out of buying one, I would buy a diesel car now if I had the chance. I do have a diesel truck that I used as my DD until we bought the Mini last month, saving us about $300 a month in fuel.
Have you put the numbers down on paper yet? once you do that you will have your answer.
Enjoy the ride