I actually had my Mk3 showing 199.9 mpg the other day. Perhaps that's as high as it goes.
I'd started from a car park at approx 500' up and, after initial acceleration, trundled down to a crossroads 1.4miles away at 150'. There was a car in front, so I couldn't have gone much faster if I'd tried.
As I approached the bottom, I saw 172.8, 187.2 and finally 199.9 mpg.