"Overgeared" is the same thing as "tall" gearing. The factory, for some reason feels that bikes that will do 150 mph+ is a good thing.:thumbup Not a bad thing either but not really too usable for day-to-day or around town riding.:confused
A front sprocket that is 17 teeth will turn fewer times as the bike is building up speed- this makes the bike "faster" at the top end but will build speed at a slower rate. The bike will not be as "quick" as one with a sprocket with fewer teeth. Keep in mind that "quick" and "fast" are relative terms.:devious
A sprocket with say 16 or 15 teeth will turn more times as the bike is building speed, and this will make the bike noticeably "quicker".
A bike that has a smaller front sprocket will cruise, at a set speed, at a higher rpm than a bike with a larger front sprocket. ie: 6th gear, 70 mph= 2800 rpm, 17 tooth sprocket. 6th gear, 70 mph= 3200 rpm, 16 tooth sprocket.
(those are not actual rpm's, this is just an example)
Here's the bottom line- Aprilia's are geared "tall" from the factory. With a 6 speed tranny, cruise at 70 mph is about 2700-2800 rpm. That's at the lower end of the power band and if you need to speed up you may have to row the tranny a couple of gears to get up some steam. Installing a smaller toothed front sprocket, on your bike will make it noticeably "quiker" and quite possibly much more "rideable" for around town and day-to-day use.:thumbup
It is one of the least expensive ways to make your bike feel (and actually be) quicker.:biggrin:biggrin You will actually give up some top end speed but I doubt if you will notice the difference between 165 and 180 mph.
Sorry to be long-winded, Gary.