What kind of difference do small airflow changes alone make to the economy of a car? The American TV show Mythbusters once experimented with this by seeing how running a car clean versus dirty would affect its economy. They took a typical US-market saloon and got it as filthy as they could, measured its economy, then gave it a sensationally good clean and tried again.
As with most practical experiments, if you tried you could pick a hole here or there in the method, but I think it stood up pretty rigorously. The filthy car returned 24mpg at a steady 65mph; the clean car returned 26.4mpg. So if you’ve ever cleaned your car and thought it felt faster or more responsive afterwards but dismissed this as placebo effect, don’t be so sure.
You had just removed weight from and improved the aerodynamics of four rotating masses, perhaps reducing their inertia notably. Mythbusters’ versions of clean and dirty were extremes, but I will bet the human mind can detect smaller back-to-back changes.
Audi, it’s worth saying, has long had form working on aerodynamics. Its C3-generation 100 of 1982, which had its efficiency figure proudly etched into its rear quarterlights (“Cd 0.30”), set new standards in the big saloon class. “Rivals’ combinations of performance and economy are suddenly made to look pathetic by this unsupercharged 2.2-litre,” we wrote when we tested it.
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