I agree - the smaller petrol engines in the fiesta are very economical and with the extra purchase cost and the higher cost of fuel, I doubt a small diesel is the way to go.
I'm after a fiesta diesel at the moment and would like to know if anyone owns a ford fiesta 1.6 tdci zetec s and are they any good or can you recommend anything else that would be economical.
Assuming you want the Zetec-S, the TDCI does 64mpg and the 1.6 Petrol does 42mpg.
A quick calculation in Excel shows, assuming 89p petrol and 99p diesel, plus a £1000 extra purchase cost, you need to do 50,000 miles before you break even. In a small car, this probably equates to 7 years or so.
Don't buy the Fiesta for doing motorway journeys, you will hate it. Anything more than an hours worth of driving in mine leaves me with a stiff back and a headache.