Why confine yourself to petrol cars?
Think about a smallish to medium car with a good 2 litre diesel.
Some experts will tell you that the extra cost of diesel is not economical unless you do more than 4000 to 10,000 miles (depends on expert).
They all miss the point that the diesel has much more torque, and when coupled to a auto box will be a whole lot easier to drive - and anything that makes life easier for us is a good thing.
Just as an example, my Peugeot 407 estate only costs about £60 a month in fuel. Some days I might do 3 miles, some days it's 20. Short runs I get 28-30 MPG. On any decent run, I am getting well over 40 mpg - you can work out the costs based on fuel prices where you live.
If you do not want anything much bigger than a Fiesta, you could look at the Peugeot 307/308, some of the Skodas (2 litre Octavia is nice), maybe a Mitsubishi, or a Seat, and there are more. But, also think about how easy it is to get in and out of - it might be OK now, but will it still be OK in 18 months? Then think about all the other "nice-to-have" features. Air-con is a luxury - but if you are sensitive to heat (and many people are) then it becomes a neccessity. Cruise control could make life much easier on your longer runs, And so on. It is not just about the fuel consumption ...