Sure, a higher gear ratio helps if you have the power and momentum to use it.
Ever try to pedal a 7-speed in 7th from a dead stop?
All direct drive systems are a fixed gear ratio which is why I am shifter kinda guy.
With little power to work with but the wind and surface resistance was low enough to compensate for drag a stock 48cc could do 40MPH sure.
For example if the rider were to tailgate a big truck like NACAR drivers do to 'draft' behind the car in front of him sure, heck a recumbent trike alone can because of reduced drag, we are talking a stock 2HP engine with big Ape Hanger handlebars, might as well add a drag shute to it for heavens sake and I'd bet it won't even hit 25MPH on it's best day.
Fine bike for those that like the look, I just don't like BS performance claims is all.