a 20T pinion to an 80T spur is 4:1 * gearbox internal (say 2.5:1) = 10:1 overall
a 20T pulley to an 80T pulley is 4:1 * gearbox internal (say 2.5:1) = 10:1 overall
There are other ways of keeping the belt in contact than tension .. a bearing on the belt surface mounted such that it didn't touch until the belt start trying to climb the teeth on the pulley but still close enough to prevent the teeth from jumping would work fine.
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