Well say if you're running at 12v and the RPM at that voltage is 6000 (ignoring the fact it will have weight behind it and will most likely not come close to that speed), with a ratio 10:1 and say 15cm wheels, you'd have a top speed of around 11mph or 18km/h and plenty of push.

Really you need to get more design info before deciding ratios. And I think you'll struggle to get sprockets in a suitable size to get the reduction you need, you'd have to use a tiny chain size and even then for a FW I think it'll be a struggle. If however you're simply using chain to transfer drive, that's a different matter.