i have 285/70 17 bfg at's and am going to put on 35" either bfg at's or pro comp at's. ive read things about putting new gears in, what does this do. if you use stock gears on 35" tires will it mess up you transmission? can changing gears also help fuel mileage? thx

 

Stock gears + giant tires = more strain on your drivetrain, including your transmission. The reason people install different gears when they add giant tires is just to get it back to the original gear ratio. You can also pick gears that can change the original gear ratio with or without giant tires. A common gear change that giant tire guys do is to pick a gear ratio that compensates for the giant tires, makes their truck quicker, and helps climb/tow easier. A common gear ratio for this example would be 4.56.

 

Read the signature. Mine took less than 6mo to kill it.

 

Taller the tire the taller the gears. I went from stock tires to 35" bfg's and it was big difference in lost power, not to mention the the damage I was causing to the drive train and tranny. However, I knew I was going to go to 4.56 gears so I wasn't to worried when I drove it for the brief period I did.

If you have a 4x4 you will need to change the front gears also...

 

how much does it generally cost to re-gear?

 

I paid $650 per axel installed for my 4.56. Well worth it if you consider the waht you may pay in the long run if you don't do it.

 

so would a move from 285/70 17 to a 315/70 17 require a regear or would i be able to get by w/ stock gears on a 99 f150 4x4. the tires would be bfg a/t if that makes a difference.

 

You will need new gears. I have the 4.6L and went to 4.56's and love them. My gas mileage is still the same and highway use is fine. 4.30's is as lw as I would go though. I paid out the door like $650 with new bearings(of course I only have a 4X2 so you would pay like double).