Are we metric or what?

We're off to Scotland to a friend's wedding in a couple of weeks. Proper Scotland. Up to Inverness then a bit further.

So in planning the trip my thoughts have come round to the car, and specifically petrol. Two things it would be handy to know are when I'm going to need to fill up (so I can try and plan on avoiding expensive motorway services) and roughly what it's going to cost.

So the car roughly has a 50 litre tank, and petrol is 90p/litre give or take a little. So far so good, the trip is going to be roughly 860 miles. So all I need now is some fuel economy figures.

36.7mpg or 7.7litres/100km

Hang on, that's no good. We don't buy petrol in gallons, and we don't measure road distances in km.

So what should in effect have been a couple of simple calculations have now become more complecated by the need to start doing some conversions between metric and imperial.

So are we metric or what?

(By the way it comes out as just over 400miles to a tank, and £96 worth of petrol)