+120rpm at the ranks you can but fastest i got is 39.6mph
To do over 40 mph, you need around double 730W, so getting on for 1500W, and that's if you are on a racing bike on the drops in a good aero position, with no head wind and no gradient. For a mountain bike with wide tyres and an upright position, you'd need a lot more than 1500W.
Maybe if you are also putting in a lot of effort, but you'd need high gearing and wouldn't be able to maintain it for long.
Downhill you might be able to go faster. But once the motor reaches it's max speed, if there isn't an increase in voltage, it won't go any faster, even if the power is available, it will be down to you and gravity to make you go faster
Useful calculators for estimating your speed on a bicycle velocity from various parameters.
bikecalculator.com
Bicycle Speed (Velocity) And Power Calculator, comparing different kinds of bicycles
kreuzotter.de