Are you using a happytime 66cc two stroke engine.
If so count on about 6500 rpm.
Here is a link to a program that may help you decide on what driven ( rear) sprocket to use.
A happy time has a built in four to one reduction before the output sprocket.
The output sprocket usually has ten teeth you decide on the rear sprocket to determine the top speed.
Anything over thirty five is hard to do without a shift kit or lots of tuning and porting , a tuned chamber, bigger carb, ignition work.
http://motoredbikes.com/showthread.php?t=18545
With a 56 tooth rear sprocket I get 22.40:1 which at 6500 rpm yields 25 mph.
With a 48 tooth rear sprocket I get 19.20:1 which at 6500 rpm yields 29.2 mph.
22.40 to 1 is very good for cruising.
This program works on Windows and GNU/linux under WINE.
I usually use ten to 40 for the first stage, leave the second stage at 1 to 1 or ten to ten. And use the final window to insert the values for the ten tooth drive sprocket on the happytime the last value is for the rear sprocket. Enter your wheel size, and the expected rpm.