That's the case for cars, but I wouldn't be certain it's the same for buses, especially large fleets of public transport buses.

All three British bus manufactures give all measurements except speed in metric, although none give an actual fuel consumption. (Just "20% better" etc.)

[1], as the first reference I can find, uses both l/100km and mpg.

[1] https://www.whatdotheyknow.com/request/fuel_consumption_of_p...

Miles per US gallon or miles per imperial gallon?

The latter, miles per UK gallon. Watch out for that, because if you don't know better, you'll read about some car in the UK and wonder why it has such astounding fuel economy!