Pedaling 60-70 rpm is too slow to be able to put out much power, for an extended time. You need to put out 75-150 watts continuous to make a meaningful contribution. Doing it at 60 rpm will mean using slow twitch, muscle fiber. This is fine for anaerobic stop light, or hill sprints. But to put out useful power for more than a few minutes requires aerobic muscle use, and a higher, lighter cadence, 80-90 rpm. Trained bike racers spin 90-120 rpm for days.
Frankly, on a bike with 2-3 kW of power, even a bike racer's 250 watt, long term cruise input, will only add 10 percent! However, making your legs go around may feel good, and deceive the authorities.