The 74mpg one will be imperial - since he is using the in-car display.
It will be 61.7mpg(US) if you convert it.
P.S. 20mph speed limits will increase pollution as people with drive about in e.g 3rd gear, rather than 5th, decreasing drivetrain efficiency. Also decreasing the ability for hypermilers to EOC. And also I haven't heard of these rules yet so they will be a few years away at least.
He sounds like he has done a lot of research into fuel economy - but is trying to present it in a simple way so people on youtube can understand it!
__________________ Team GasMisers5 - #1 for first three rounds of the original GS Fuel Economy Challenge
Miles displaced by e-bike since 1 Jan 2008: 62.6 (0 kWh used)