Hi Anthony,

The 20,000km changeover point should be seen as a guideline rather than a strict number and bear in mind that, regardless of mileage, the diesel engine should still return better fuel economy. The reason we say a petrol engine is better for lower mileages is because of the bigger picture: a diesel car usually costs more to buy and comes with much higher maintenance costs, especially if it has been used in predominantly low-speed driving. Modern turbocharged petrol engines and hybrids are certainly closing the gap to diesel in terms of raw fuel consumption so even with annual mileage of around 20,000km, the diesel only makes sense if you're on higher speed roads a lot of the time - that's where they really come into their own. 

In terms of resale value, it's a tough one to call, as we don't really know what the government is going to do about diesel taxation (if anything). I don't see a drastic decline in demand for second-hand diesel cars in the short-term, however.