Changing gearing J bike
Why this mod?
When people change the gearing on their bikes, it's usually because they want lower rpm for highway riding. This is achieved via a larger front and/or a smaller rear sprocket. Sprocket sizes are shown as the number of teeth they have, so a 41-tooth rear will be smaller than the stock 45-tooth. The first gearing change many people make is to put a 15-tooth up front.
The EX250J (08-->) has the same sprocket sizes as the older F model, 14/45 stock, so changing sprocket sizes will yield similar results. See the FAQ page on changing gearing and other gearing info in the Final Drive section.
Differences between the 2 models
There are some differences in engine characteristics between the two bikes, but changing sprocket sizes will affect your bike the same as on the older model. The wheel size is also different between the J and F series bikes, but the actual diameter of the wheel and tire is nearly the same. The 130/70-17 BT45 rear on the J bike is actually a negligible .2" shorter than the stock 130/80-16 K630 on the F.
To find part #s for other manufacturers, go to the FAQ chain & sprockets page and look them up on the manufacturers' sites.