Not necessarily - although smaller diesels rarely make much financial sense. That means supermini buyers probably shouldn't even think about one.
Take the Citroen C1, for instance. The diesel version does an extra seven miles to the gallon and only costs 1000 more than the petrol. Sounds tempting, doesn't it?
Well, unless you plan on using it for space travel, forget it. You'd have to do around 1.3 million miles just to make the extra cost back. That's nearly three times to the moon and back.
A petrol, rather than diesel, Ford Focus will be the cheaper alternative for most people, too.
A petrol Mercedes E-Class probably won't be, though. You'll have to do 61,000 miles before you make back the extra cost of the diesel car in fuel savings, but many owners will spend a lot of time on the motorway, so they could do so quite easily.
Take into account all the other costs involved with running a car and the diesel E-Class will save you nearly 1700 over three years - more if you do high miles. That's mainly because the petrol Mercedes will lose its value quicker than the diesel