40-45 is probably not a realistic goal for a China girl-
Mine, with light narrow 700c wheels and a 34 sprocket might push 35 if I really pressed it- I don't- the motors are notoriously unreliable, while my bikes are like a second car- I want long performance out of them- not break neck speed.
Granted, I'm only running a stock NT Speed carb- so a change there might yeild me some more screaming RPMS, but I feel like I'm pushing the limits on a bike anyway at that speed- and I raced USCF fourteen years road and track in my crazy days. Plus my bikes are set up with as much alloy and titanium as practical- but also baskets racks and bags to be able to carry stuff.
I usually cruise out about 25 on my analogue speedo- probably not too far wrong- , but can get that up to 30 to stay with traffic if I want- I ride them slower all the time- at 60 I want to just cruise around the lake on my John Candy pontoon boat and enjoy life staying alive.
My 50 cc cruiser- set up much the same otherwise is about 5 mph slower all around, with a 39 alloy sprocket- and it's all flat here- a few years back I was getting up near the tops of the steepest mile long climbs in southern Indiana before bogging down on a 36 if that helps- all on standard kit carburetion.
We're running with the shadows of the night.....