I'm trying to work out whether to buy a petrol or diesel Ford Mondeo, taking into consideration that the initial price for this car secondhand is the same, say £2000. If I'm doing a mix of short five-mile journeys during the week, and 200-mile blasts at the weekend, and with a petrol being 10mpg less efficient than its equivalent diesel (but 5p per litre cheaper to fill), what's the better buy? And secondly, which is the more reliable?