With China girl engines they which are actually 66cc they generally run a total reduction between 11:1 to 14:1. The engines that run a reduction under 5:1 are the 212cc.
As for gear ratios a 9t is around the smallest you'll find that'd be practical to use. I recommend this engine to jack shaft 10:40; jack shaft to rear wheel 10:40 for a total reduction of 16:1.
Here's your speed formula:
RPM÷(1056× total reduction)×wheel diameter × ⚻
6000÷(1056×16)×27.5×⚻=30.7 mph.
That's a very realistic build for a first time DIY build. I also suggest for the small sprockets to use 410. This way you can use 410 chains.
As for gear ratios a 9t is around the smallest you'll find that'd be practical to use. I recommend this engine to jack shaft 10:40; jack shaft to rear wheel 10:40 for a total reduction of 16:1.
Here's your speed formula:
RPM÷(1056× total reduction)×wheel diameter × ⚻
6000÷(1056×16)×27.5×⚻=30.7 mph.
That's a very realistic build for a first time DIY build. I also suggest for the small sprockets to use 410. This way you can use 410 chains.