i have a completely stock 2005 400ex and do a lot of trail riding if i go with a bigger 22" tire should i decrease my teeth in my front sprocket to 13 and increase my rear sprocket to like a 39 to increase the acceleration/torque for trail riding? i dont need a lot of top end speed

 

The 400ex is a great trail quad and can be lugged down in any gear.
If I were you, I wouldn't bother changing the gearing, the 400ex can handle the 22" tires without a gearing change.
If you feel you need to gear down after riding it on the trails, just drop a tooth on the front and leave the rear alone.

 

MY old 400ex had 22'' mud lights and i believe that it was geared down a tooth. on only trails i agree with red400exrod. but if you plan on ridding sand or even trails with alot of turns were you can do without some extra top end it's a good bottom end moddifaction.