While the question you've asked is quite simple, I'm afraid the answer is quite complicated. Fuel economy should not be your only concern here. For a start you need to be cautious of tax. The E39 5-series was in production from 1996 to 2003 - which means it crosses two tax bands. Cars registered between 1 March, 2001, and 31 March, 2017, pay a rate of tax based on fuel type and carbon dioxide emissions. Cars registered before 1 March, 2001, pay a rate of tax based on engine size (under 1549cc and over 1549cc). So what you may gain in mpg by buying a late diesel, you could lose in tax. You also need to consider reliability issues - BMW's six-cylinder petrols are often described as bulletproof, while this era of diesel engines can suffer from reliability issues. There's also another issue to be aware of - BMW's aluminium M52 six-cylinder petrol used nikasil cylinder liners - this was an issue because in the 1990s, petrol contained a high sulphur content - and sulphur caused premature bore liner wear. Once the issue was spotted, the cars were recalled and replaced with steel cylinder liners - but you'll need to check the work was done. So the takeaway here is to decide what car you want and do your homework. Yes, the E39 is a great car and we prefer it over the Mercedes E-class - but if you want something German, prestige, and rear-wheel drive, you need to drive both and see which is right for you.