I just got a 2011 ninja 250r to ride but the gas millage is not the best. Im getting 40 miles to the gallon. I have to ride 15 miles at 70mph and im wanting to get better gas millage. I re geared the front sprocket to a 15 tooth but what else can I do??
40s not bad mileage to get, but is low for that bike. If you're not sure on maintenance, a full maintenance service might be due, air filter, oil/filter, spark plugs, tire pressure, etc. Get a service manual (available on the Internets), check daily maintenance checks, and mileage/annual service intervals and make sure everything's up to snuff.
If it's not a maintenance issue, your weight can affect that bike drastically. If you're a heavier person, that could be what's making the difference. Riding habits can also affect mileage.
Aftermarket mods could make a difference too. Depending on if the bike has mods, as you mention a change in final drive ratio, make sure you understand how these changes/modifications affect fuel mileage vs. a stock bike.
General thoughts on it from ninja250 org, pretty much reflects what I stated. You're either missing something regarding maintenance, it's your riding habits, it's you (physically), there's something wrong with the bike, or factoring your current riding loop and habits results in your current mpg. 40 mpg is basically ninja250's org low end of mpg expectation, so it's not unreasonable if the conditions of your riding merit that mpg.
You have a small engine that is trying to haul a lot of freight......and it's pushing a lot of air at 70 too.
It NEEDS to turn a lot of RPMs to develop the horse power necessary.
You will be sorry if you change the gearing any more than you have already. Same for screwing around with the engine.
If and when you just can't STAND the way the bike runs or the gas mileage that you are getting........trade it in for a 650 or bigger.
And to repeat: You seriously should NOT mess with the intake or exhaust. You will be sorry.