What kind of gas mileage can I expect?
The Ninja 250 is a relatively fuel-efficient bike. Most riders report between 50 and 70 miles per gallon (between 3.36 and 4.7 litres/100km). With aggressive city riding, mileage can go as low as 40 mpg (5.88 l/100km). With gentle freeway or open road riding, it can get as high as 75 mpg (3.14 l/100km).
If you plan to ride primarily in-city, expect between 40 and 50 mpg. If you plan to ride primarily on the freeway, expect 55 to 65 mpg. If you plan to ride a mix of both (which is what most riders do) expect between 50 and 60 MPG.
The 'new' EX250J has slightly lower mpg numbers, but it is still a very fuel-efficient machine.
Fuel mileage is heavily affected by the following factors:
In city riding, weight and riding style have the biggest effects. More weight and more aggressive riding will get lower gas mileage than less weight and gentler riding. In steady-speed riding (such as on a freeway) speed and wind resistance will have the biggest effects. Wind resistance goes up with frontal area (your silhouette when viewed from the front) and speed, so if you're a big person who sits upright and goes fast, expect lower gas mileage than someone who crouches over and goes the speed limit. Any headwind will decrease economy, while a tailwind will increase it. Note that wind resistance goes up with the square of the airspeed, so 80 mph has about double the resistance of 60 mph.
Generally speaking, engine modifications for power (Muzzys exhaust, K&N pod air filters, etc.) are not intended to increase gas mileage, although for some riders they have that effect. Do not perform these modifications if all you want is increased gas mileage. The Ninja 250 is already set up for excellent efficiency.
A larger countershaft (front) sprocket (a 15 tooth) lowers engine rpm at a given speed, which increases fuel economy at the expense of acceleration. Higher tire pressure typically leads to better fuel economy, although the tradeoff with traction can be significant and it's not worth saving a few pennies on gas if it makes the bike less safe.
If the engine is out of tune (carburetors unsynchronized, valves out of spec, dirty filters, etc.) that usually has a negative impact on economy. Sometimes this impact can be significant, although the bike won't run well if it gets very far off. Note that an incorrectly adjusted drive chain can noticeably decrease fuel economy as well.
Bottom line: Don't expect 70 mpg out of the Ninja 250 unless you're riding in perfect conditions and can reign in your throttle hand. There are numerous things you can do to increase economy, some of which are quite painless, but the economy you get out of the bike is ultimately up to you.