Your gear ratio is simply the rear sprocket teeth divided by the front- 72/6= 12- so with that alone, it's 12:1. The wheel size changes the effective gear ratio when compared to the same sprockets on wheels of a different size, but it does not actually change the gear ratio. What I mean by the convoluted sentence is that your 12:1 is 12:1. However, because your wheel is 6" smaller than a 26", the wheel is 23% smaller than the 26", therefore, with a 20" wheel, your setup acts like a 26" with 23% lower gearing.
Your 20" at 12:1 acts like a 26" at 14.76:1. That's usually not quite enough to get the engine in the power band. You probably want a reduction of closer to 19:1 on a 26" bike which is equivalent to 14.63:1 on a 20".
