You change the front sprocket up 1 or 2 tooth. However you'll loose torque. I'm not sure if it can be geared any better for FE. If the gear ratio is too high then you'll loose top speed since the motor isn't strong enough.
Gears cost about $15.00 and take about 20-30 minutes to change. Common practice on bikes.
to re-gear you grab your socket set, unbolt the rear wheel, unbolt the sprocket, bolt the new sprocket on, bolt the wheel on, adjust your chain, and ride home from the store that you bought the sprocket from.
stock ninjas from 1986-87 had a rear sprocket of 42 teeth, all ninjas come stock with 14 tooth front sprocket, and 1988 and newer all have 45 teeth in the back.
A Parts Unlimited brand sprocket (Dennis Kirk catalog), sells both the 42 and 45 tooth light weight sprockets for $28.99 each, and a non lightened 50 tooth (giving you higher revs for quicker takeoffs) for $20.99, front sprockets wear faster and are avalible from this catalog in the 14 tooth stock size for $12.99
Given that you've got a later model, you've probably got a 14/45 sprocket set up.
You can change the front sprocket with a 15 or the rear with 42 to come up with the same ratio of 3.00 compared to the OEM ratio of 3.21.
Changing the rear sprocket will make it so that you have to adjust the rear axle more than you would by changing the front sprocket. Whether you can get that with the OEM 106 pin 520 chain? That has to be something you look at or you'll need to replace that also.
Replacing the rear sprocket's easier. You're really just going to remove the rear wheel, replace the sprocket, then install the rear wheel again, and adjust the chain properly.
Replacing the front sprocket will require you to remove the sprocket cover and any associated hardware. Might need an impact wrench to get the nut off the countershaft that holds the front sprocket on.
I haven't found, personally, that front sprockets wear out sooner in my applications. We do a lot of rear sprocket changes, and I will wear out a rear one before we do in a front one even though the rears aren't seeing as much mileage.
Comparing sprockets, a larger sprocket doesn't require the chain to make such a change in it's radius that it's following. So, using a larger sprocket on the front can reduce that load of the turn.
Will the gear ratio change improve your mileage? That's a good question, and its one that you'll have to think about. If it were a bigger bike, making a change of that kind would probably relate to about 600 to 900 RPM's. How much it will change on the 250, I don't know.
I regeared my 2001 YZF600R (more torque than the R6) ...I did the full DID 520 change over also. I put a smaller front sprocket by a tooth and it made a world of difference driveability wise...much smoother. It also helped FE. The rear sprocket I had to change because the 520 chain is a different size but I left the same gear size on the rear. I could basically go back and forth changing the front sprocket depending on how I wanted to ride (younger then) and didn't have to do much else than adjust the rear wheel forward or back a bit.
2006 Jeep Liberty CRD...Founder of L.O.S.T.
OME 2.25" Lift w/ Toyo Open Country HTs 235/75/16s
ASFIR Alum Eng/Tranny/Transfercase/Fuel Skids
2002 Air Box Mod...Air Tabs (5) on Roof...(3)each behind rear windows
Partial Grill Block with Custom Air Scoop and 3" Open Catback Exhaust
Lambretta UNO150cc 4 Stroke Scooter