I was thinking of this.
I can power my f4i up in third though, so i'm not sure how much i need it...
What's it do the the gas mileage?

 

well, it really is all relative. Your bike doesnt have any more power, its just moved down in the rpm range, so it feels like it has more power. example, coming out of a corner at 8-10k rpm feels very strong with the sprocket change, to get that same feeling with stock gearing you need to be closer to 12k.
same power, just the gear ratio is changed so you have to up the rpm's bit with stock.

On the flip side, with stock gearing on the interstate, running 80mph, you may be at 7-8k (my numbers may be off - just using as a reference). but if your gearing is down, then to run 80mph, you may need to be 9-10k rpm. So, since you are running more rpms, you are also using a little bit more gas.

also, if you dont correct your speedo, your odometer and speedo will be off. bike will register that you have gone 120miles, when you really only went maybe 110, addind to the feeling that your gas mileage is out of whack.