WTF? 300+ MPG diesel bike?

[electro]

this dude made probably derpest looking diesel bike, specs:
5hp texmo engine; max rpm = 2000
transmission: fixed single gear (no cvt or gearbox)
a belt and pully setup for transmission
top speed: ~50 mph
mileage: ~305 MPG

the video: https://www.youtube.com/watch?v=bXXJ6mqvEP0

how it can be this efficient with no cvt or gearbox, belt, small tyre?

i know 5hp engine uses less fuel and weight of bike (and biker) is not much but still, 300+ MPG sounds too much, HOW?

[Stuart]

300mpg sounds about right for 5hp. Only good for tiny scooter like machines though.