As with all cars of that age, the diesels are probably well into intergalactic miles, probably been mis-fuelled several times, and a DMF failure waiting to happen.

I drive a Mondeo that's got Mazda's 1.8 petrol MZR engine fitted, same as you'd find in the 6. I get 33-40mpg depending on how much town driving I do, and there's little to go wrong. It's a solid little engine, and goes pretty well. 125hp rated, most seem to output a little more.

To be honest, the engine is the best bit of the car on the Mondeo...

Most posters seem to say under 12k a year petrol, over 12k diesel.

I don't think anyone can say for definite which engine will be cheaper to run as that depends on too many factors.

However, I used to own a 2004 Mazda 6 petrol so I can give you my experience. Servicing was cheap (independant garage) as it never had any major problems. Reliability was rock solid, it never let me down once. Economy wasn't very good, I averaged 27-30 MPG in mixed driving, but I do have a heavy right foot. General wear and tear parts like tyres, brake pads etc were no better or worse than any other car I've owned of similar size.

I would definitely recommend checking for rust as this model has a few issues in that area and my car suffered very badly from it. If it didn't I'd probably still own it.

For what it's worth, a common experience for me was telling mechanics I know what car I was driving and seeing a look of concern on their face. Then a look of relief when they found out I was driving the petrol rather than diesel. I'm sure there are plenty of reliable Mazda diesels out there but there are enough dodgy ones to have built up a bad reputation.

I think the new 2.2 is supposed to be much more reliable but personally if going Mazda I would still pick the petrol.

Personally I would drive an example of both. Some drivers seem to prefer a revvy petrol engine and others the more relaxed slogging power of a diesel. I hate stopping for petrol and the buzziness of a lower geared petrol car on the motorway so i'd probably take the risk and buy the diesel.

Diesels have issues with the reliability of expensive components but you can stack the odds in your favour by buying a lightly used well serviced car with a warranty if possible and being strict with oil changes and servicing . Most cars that are well looked after don't have any major problems.