If you read Autocar magazine regularly, you may know that we’re running a Honda Civic Tourer on our long-term fleet. It’s powered by Honda’s new-for-this year 1.6 i-DTEC diesel engine, and it’s proving to be spectacularly economical. I like it a lot.

It regularly records an average economy figure in the low-70s on my 36-mile commute to work, and mid-60s is easily achievable in everyday driving. Even with regular town work and use by considerably more throttle-happy drivers than me, the overall average currently stands at 59mpg after 5000 miles. That’s pretty good going for a 4.5m-long estate with a 1668-litre seats-folded load capacity.

A couple of days ago, a Civic hatch arrived at the office, sporting some extra driver-assist safety kit for us to try. Again, it was a 1.6 i-DTEC-powered car, but in SR trim rather than our Tourer’s EX Plus spec, which meant that the hatch arrived on 16-inch wheels while the Tourer runs on 17s. The hatch also had 2700 miles on the clock, about half that of our Tourer.

Before we go any further, a few caveats. Aside from the Tourer’s 59mpg overall average, which is worked out using some meticulous notes and a calculator, all of these figures are taken from the two cars’ trip meters. 

By our maths, the Tourer’s trip consistently over-reads by four per cent, so 70mpg on the trip is actually 67mpg. That’s assuming the miles it records are the same length as everyone else’s miles. I haven’t calibrated the odometer, but I don’t think many people do when working out their day-to-day economy figures. And anyway, the Honda says my trip to work is the same length as usual, so I don’t think anything untoward is going on there.

I also have a particularly efficiency-friendly route to work, the details of which you can read about here, but in summary it is (a) nearly all motorway, which tends to flow at around 45-60mph of a morning, and (b) downhill. In the evening, the traffic tends to be (c) more stop-start and (d) it’s uphill.

So anyway, I took the hatch home that night and was slightly surprised when it recorded a 69mpg average for the trip in particularly heavy traffic. Then, the next morning, I was flippin’ flabbergasted when it claimed 91.4mpg for the journey back to Teddington. In fact, at the point on my commute at which the rolling total for the average economy generally peaks – leaving the anticlockwise M25 and joining the London-bound M3, 23.5 miles in – it was up to 97.5mpg, before dropping over the final 11 miles. By the time I got to the office I was almost disappointed that the average figure was ‘only’ in the low-90s. 

Those are pretty astonishing numbers, but what really confused me was why the hatch was so much more economical than the Tourer over a route I drive every day. 

The hatch is lighter than the estate, but only by a claimed 14kg (1416kg compared to 1430kg). There must also be a difference in drag coefficient figures. Honda doesn’t release the numbers, but the differences in claimed combined economy and CO2 figures suggest that the hatch is more slippery.

Honda claims 78.5mpg combined (is that all?) and 94g/km of CO2 emissions for the hatch, on 16-inch wheels, and 72.4mpg and 103g/km for the Tourer, on its 17s. (For the record, a hatch on 17s is rated at 76.3mpg and 98g/km, a Tourer on 16s is 74.3mpg and 99g/km)