The Puzzler

New feature. [and you didn't discuss it with me, the editor? Naughty boy - The Pole]. A weekly puzzler for you to consider. I will set a question each week.

This week's is a Metric versus Old Imperial measurement. As you all know the rest of the world is all virtually metric, but what one factor on modern cars, even in metric countries, is still measured Imperically i.e the good Old Blighty way?

Think about it.