Yes, you could reach the speed of 45 mph, 55 mph or even 65 mph on a 125cc scooter, but don't plan on sustaining those speed without shortening the engine life.
You wouldn't consider driving your car at 140+mph for extended period of time to put that kind of stress on your car engine would you? Similar stress if not more is done to the scooter engine while operating at 55+ mph.
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I've owned my Vino 125 for almost 3 years. As to the question of 96mpg, fuel consumption for my last 5 fill-ups are: 93.42mpg, 102.09mpg, 91.75mpg, 91.35mpg, and 97.05mpg. I weigh around 146, and the driving terrain is the Sierra foothills. The lower mileage (in the low 90s) reflects commute driving where there are a number of stop lights in the 8 mile route.
I've learned that to get this mileage I had to adjust the way I drive to anticipate stop lights so I coast to them rather than follow most drivers who choose to zoom up to red lights and wait.
I purchased a Scangauge II for my Honda Accord and that has certainly helped me adjust my driving to improve mileage on the car.
I have had my Vino 125 for a couple of months now, and I like it pretty well. I only see 80-85 mpg, but then again I drive it hard and I am 195 lbs. My top speed is 55, although it will hit 60 with a slight downgrade. I think I am going to upgrade to a 150 or maybe even a 250 of some sort next spring. Like others have said, it works well if you only have to do 45 mph. If your drive includes anything faster than that, I would go bigger.