30 teeth on the 60 as it would be a 2:1 reduction ratio, this way you will achieve more output speed but be sacrificing the torque multiplication factor. It would be like moving a standstill car from 3rd gear. It can be done but your engine should be very very torquey.
20 teeth will give you good torque multiplication but have lower speed as it is a 3:1 reduction ratio - more like first gear of a car excellent instant very strong movement but lower top speed.
go to a gunna (sugarcane) ka juice machine and see what I mean - the large wheel is run by a belt being spun by a 2 hp motor with a tiny pulley, the reduction ratio is like 20:1 - massive torque to crush the gunna between the presses. Not much speed though. You can replace the little motor with a 400 hp cummins diesel and reduce the main pulley dia considerably to speed up the juice output.
btw take my advice with a pillar of salt - I was usually kicked out of physics and chemistry labs because of my adventures in them.
Use bevel cut instead of straight cut gears, it ensures more contact area to transfer the force.