I've fitted a new set of wheels and tyres to my Audi A2, smaller ones than before, and improved its fuel consumption notably.

"I reckon it would be pushing 80mpg behind a lorry at 55mph," said a colleague when I told him how the trip computer readout (measuring over the duration I've owned the car) had climbed from a steady 68.8mpg to 71.8mpg and rising since the new hardware was fitted. I think that's a considerable underestimate.

Since making the wheel swap, I had put the difference down to narrower and more efficient tyres, plus the fact the new wheels were lighter. Then I read some quotes from a man with an ever more important job. Moni Islam, Audi's head of aerodynamics and aeroacoustics, recently told Autocar: "The A2 pushed the boundaries of car design with its proportions based on the work of [famous 20th-century car designer and aerodynamicist] Wunibald Kamm, and its optimised underbody and wheels."

I had so concerned myself with width, which affects the car's frontal area (which you multiply by the drag coefficient to find overall aerodynamic drag), and the friction of the tyre itself that I hadn't considered the aerodynamic effects of the wheel design in the improvement. I've gone from nine thick, sharp-edged wheel spokes to basically a disc with 10 discreet holes in it, so of course each wheel will be spinning less turbulently than its predecessor.

And because the wheels are sitting more inboard the body, they're even less likely to upset airflow down the sides of (and underneath) the car.