Brains, let's test these gear ratios. Use an 8-tooth drive gear and 74-tooth sprocket, so you can make adjustments using 7t and 6t sprockets. With 8t, you have 9.25:1 gearing. Now create a jackshaft, using 36t sprocket on the hub and 16t on the jackshaft. Jackshaft ratio is 2.25:1. Multiply by 9.25 gets you 20.8:1. Using an online gear ratio calculator with 20" wheel gets you 64 kilometers/hour(40mph) at 14,000 rpm. With a 9t, speeds are 72kph(45mph) at 14,000 rpm.
For more low end, install 7t sprocket, which gets 10.57:1. Multiply with jackshaft 2.25 results in 23.78:1, and 56kph(35 mph). For even more low end, install 6t for gear ratio of 12.33:1. Multiply by 2.25 for 27.74:1 and speed of 48 kph(30mph).
All speeds at 14,000 rpm.
If someone can suggest an online calculator, Brains can work out more gear ratio combinations. It's good to start with 8t drive sprocket, because you can adjust up to 10t and down to 6t.
Good luck.