Ya your right you need to go 5-10mph faster then speed limit to be safe in traffic
What is your setup (kit/cylinder type and cc, pipe, carb, clutch etc.) and how much power do you think you make?
If your bike is kitted it should be able to pull 17x40 gears (Changing countershaft engine sprocket is easier then rear wheel sprocket) 16x36 maybe too much gear,
Most mild 70 cc piston port setups I do with 14 mm bing, circuit/estroil pipe with tuned clutch and jetted and tuned correctly can pull 17x40 gears and hit 47-48 mph with good takeoff and do ok on hills,
But that’s me, the speed you want depends on your setup, how good tuned you are, your powerband, (how much hp and torque you make at what rpm) and how tuned your clutch is,
This may help you figure gearing but you still need to make power and tuned clutch to pull taller gearing,