My tires got cracks and my chain has rust. :nono . So now im looking to do some mods. But before i start buying stuff i want to know some basic info.

I have been told to get the rear sprockets replaced if you are changing out the chain. But im not even sure what 520 or 525 really means. Im also looking to get better mileage and so i was thinking getting a 15T/42rear.

If I remember it correctly, that's the width of the chain. Wider=stronger=heavier. Most people would go for lighter chain because it reduces the unsprung weight and rotational mass, which suppose to help with acceleration.

Thanks! and from what i have read that the stock chain on our bikes is 520.

Some people will swap out the heavier factory chain on bigger bikes like 600's but they end up having to replace the those lighter 520's more frequently as they ware and stretch out and under the loads exerted by the heavier more powerful bikes.

520 chains are typically found on dirt bikes which can easily weigh 60% of a 600cc sport bike.

As for the sprockets I would probably stay with the factory ratios. While you might be able to improve your mileage it will be hard to determine as your speedometer will no longer be accurately calibrated to these new sprocket sizes and if improved mileage is your goal I would suggest good throttle management habits. If on the other hand you were going to try to improve top speed or acceleration then a gear swap would be the way to go.

It's not like ninja 250's blow the world away with its power, so it sounds like a smaller chain than a 520 would be fine in terms of reliability, right?

As far as sprockets go, couldn't you find a combination that will actually increase your mileage?

It's not like ninja 250's blow the world away with its power, so it sounds like a smaller chain than a 520 would be fine in terms of reliability, right?

As far as sprockets go, couldn't you find a combination that will actually increase your mileage?
520 is as small as it goes. ours are 525 stock.

yes you can change sprockets for better mileage but will sacrifice acceleration. Like Freakdawg said throttle management is best practice

Oh man, I think acceleration is the last thing you want to sacrifice on a 250r...haha

Ok, so it sounds to me like a 520 chain with stock gearing is the way to go.