Do you have an tuned pipe ? if not.. i'll sugest you the Jet Pro V3.. to get the craziest top end and very high rpm revs..
If you keep 17/57.. you will probably do 42mph..
With 18/55.. you will hit 47mph maybe
there is a thread on the forum who will show you how fast you can ride
My Brother hitted 52 mph with 20/54 .. riding the 26cc stock engine and a Jet pro V3 .. but with a 5T gear & 5b tires..
On the road, it might look slow but if you riding outside every where... you will be happy