At the bottom of this piece you'll find links to the first three parts of this series explaining where we went and what we saw and all that, but one of the primary reasons for doing this road trip was to test largely equivalent petrol and diesel versions of the same car against each other. The BMW 3 Series is ideal for the job. It's comfortable, handsome, desirable even in its lowliest guise and genuinely great to drive. BMW Ireland kindly loaned us a 316d and a 316i, both in SE specification.
Now we'll admit that our road trip was far from scientific, but we did set a few ground rules, such as always sticking to the rules of the road. It was also agreed that there would be no nonsensical 'hyper-miling' to eke out the maximum economy. Instead, each team of drivers would drive normally, in convoy as much as possible, to see just how much more economical the diesel car would be. After all, it's over €2,000 more expensive to buy than the petrol model.
The 316i covered 679.3 kilometres and required 43.37 litres of unleaded at the end to 'brim' the tank. That equates to an average consumption figure of 6.38 litres/100km (44.3mpg).
After 678 kilometres the BMW 316d drank 36.67 litres of diesel, which means it used 5.41 litres/100km (52.2mpg).
No major surprises there, though it's worth remembering that we spent little or no time at a standstill. Both cars' figures are impressive, though the 316i's average may surprise some that automatically shy away from petrol power.
2013 road trip part one: best laid plans