The 48V 5kW motor with the 8:1 gear ratio axle would need at least an 18" outer diameter tyre in order to reach 40 km/h with the motor running at ~3,740rpm.
From the above diagram, the force required to move a 600kg vehicle up a 10 degree slope would be at least 105kg
(600 x Sin 10) and that's without allowing for frictional forces at the tyres, mechanical losses through the gear-train and wind resistance etc.
According to
my spreadsheet, the motor would need to develop at least 30Nm of torque before the vehicle would even move, let alone accelerate up the slope.
The motor draws roughly
9Amps/Nm @ 48V, therefore 30Nm x 9Amps = 270Amps
(13.5kW).
This seems to be a lot of Amps for a 5kW motor.
Alan