I want to change my gearing
How do I change my gearing?
For about $50 you can get some new sprockets for the Ninja 250 that will lower the RPM's at highway speeds (better mileage). On a stock Ninja the front sprocket has 14 teeth and the rear sprocket has 45 teeth.
The first mod done by many people is replacing the front with a 15-tooth sprocket. If that doesn't drop your rpm's enough for you, you can try a 44-tooth or a 42-tooth rear.
There are a couple other options. The JT catalog lists a 16-tooth front. Don't Do It. It will cause clearance issues. It's better to step down in the rear than go any bigger than a 15 up front. You can use a 41 -tooth in the rear if you do lot of commuting and don't have to worry about going up hills. Several members on the board have a 15/41 combo, but they are shooting for gas mileage, plus they live in flat places like North Texas.
See the chain & sprockets page for information on available products.
Does installing a new front sprocket make a significant difference?
Changing the countershaft gear from a 14 to 15-tooth sprocket is the most common gearing mod, and it does make quite a difference. At first the bike might seem sluggish after the change; the torque you were used to isn't really there. As you ride around town for awhile, it may seem that the bike isn't revving as quickly as it used to- this of course is to be expected, since the countershaft is now working harder. But the change in acceleration is really marginal; though the bike now takes longer to rev through the gears, each gear is putting out 7% more travel per revolution.
So the twist on throttle has to be a little greater now to get a satisfying 0-60 run, but the high-speed cruising is noticably improved. Until you get used to it, you may think you're in sixth at times, when you're really still in fifth. This mod also makes first gear seem more useful.
It isn't too difficult a task: loosen the rear wheel, remove the countershaft housing, remove the 14-tooth sprocket, put the new sprocket in, torque the retainer bolts, replace housing, set correct tension to chain and torque rear wheel, and clean/wax the chain. The sprocket switch takes about an hour overall; just follow the process as outlined in the service manual. Popping the old sprocket off is a snap; putting the new one on takes a bit because the countershaft teeth are cut to very close tolerances. Cost= $15.
Is there any advantage to installing a smaller front sprocket?
Some track racers have used a 13-tooth front for a little while. It makes it harder to keep lube on the chain because the chain is turning a tighter corner. Also, the chain eats into the keeper, even if you file it waaaay back.
The only advantage is a) lower first gear b) puts ratios closer together. Aside from that, it's all the same because you end up in 3rd when you would normally be in 2nd... etc.
What drive ratios are attainable?
The most common non-stock sprocket combinations are:
15/45 Replacing the stock 14-tooth front.
Information & opinions on some gearing combinations
13/47 = .27660