I have 96 two wheel drive. Runnibg 385/75/16 gettiing ready to put on 305/75/16. Would it be a good idea to use the 4:10 gears in the rear end. Will they still work with my possi-traction??

 

I think you meant a 285/75/16. In my opinion, you would want at least a 4.10, but a 4.27 would probably be better. 4.10's are said to be the ideal gear for 33's, the 305/75/16 is a 34" tire.
You should be able to keep the posi too.

 

What tires did your truck ORIGINALLY come from the factory with? It should be on the sticker inside of the driver's side door frame. I'm pretty sure it didn't come with 285s from the factory. IIRC, those were only on the OFF-ROAD models. To figure out what gears you should put in for this swap, you first have to know how much performance you lost in going from stock tire size to 285s (there's a formula to figure all this out). Also, what gears do you have right now?

 

truck cam factory with 245/75/16
don't know what types of gears are in there right now, just what ever came from the factory.
right now I already Have 285-75-16 going to use 305/70/16 tires

 

Keep in mind that just changing gears to compensate for diameter change won't retain performance - the extra mass will slow you down too.

 

So what all modifications would you do if you wanted to maintain normal proformance aith those tire?

 

Wiser minds than mine will give you better advice. But you're changing "unsprung" weight considerably by increasing to 305's. So your gonna have to spend some money on suspention parts. A '96 probably has at least 85-90,000 miles on it. I used to rebuild engines a long time ago. Every part in the engine wears proportionately to the rest of it's parts. If you start adding the serious preformance stuff your gonna need to gain the necessary H.P., somethings gonna give.

 

Well...assuming you have 3.55 gears, you would have to go with 3.91/3.92 gears just to get back to stock performance with your new tires. That includes the performance you lost going to 285s....as well as compensation for more rotating mass.

Also keep in mind that when changing from numerically lower to higher gears, you often need to change differential cases as well.

 

3.92 gears would not be low enough for that size tire in my opinion. You would not be back to stock at all, that is almost a 34" diameter tire. 4.11 is closer to stock with that size tire.
Also, you won't have to worry about changing differential cases until you get over 4.56.

Your stock suspension should be able to handle those tires, but you will notice a difference in ride quality. Beefing up your suspension is not a bad idea if for no other reason then to help improve the way those tires ride and respond on and off the road.

 

You're also going to find your brakes won't work as well. Switching to braided hoses cuts down on the rubber expanding when it gets hot, reducing some spongyness in the pedal. Switching to a dot4 fluid will also help if you're running dot3 now. dot5 isn't always compatible with the seals or something, but dot 4 is and it's still got a higher boiling point than the dot3.