I have 308 gears in my 86 fullsize blazer. Can I put 38" tires on without chsnging the gears. I only do trails when I go offroad.  Thanks Phil  

with that big of tire you definatly have to switch gears. I would put at least a 5.13  

Lemonhead:
with that big of tire you definatly have to switch gears. I would put at least a 5.13

Yes it has to be a slug at times with 3.08s and even near stock tires. 5.13 would be good here. DO not lock axles if they are stock and you want to give them half a chance of lasting a bit. Realistically I would limit it to 35's with stock axles. (33's with lockers) if you want to okay with axles when you play hard.  

I’m not to fond of any thing lower then 4.88 my brother-in-law snapped two pinions that were some were in the 5.13 area once you get past the 4.88 mark the pinion gets small and is week.  

The point at which pinion size is a issue depends on rear axle. If you snap a 5.13 you would snap a 4.56 delivering the same amount of torque to ground because 4.56 would require a higher input torque to get same ground torque. Material used in gears matters too and you do not want to use cheap ones with 5.13's either.