rear end gearing
 

i have a 93 f150 I6 4x4 3.08 open diff 5spd. i just put a 4" lift and 33x12.5 tires on it. with stock tires i was at about 2100 rpms @ 70mph. now im at about 1600 @ 70mph. i cant hardly drive in od on freeway going up inclines without losing speed and having to downshift. what gearing do i need to get it back to what it was or close to it. i was thinking 3.73s or 4.10s. has anyone else done the same thing?

thanks

http://www.therangerstation.com/tech...tioChange.html

Here is a link to a calculator.

Your original tires were 29"; the new ones are 33".
An approximation of what it would take to restore your equivalent gearing:

(33/29) x 3.08 = 3.50

"close enough" to 3.55.

That's really not much gear though, you may want to consider bigger gears to give you more reduction in the lower gears at the cost of a bit of highway fuel economy. Your idea of going to 4.10s would give you the approximate equivalent of 3.55 gears with stock tires; 3.73s would be equivalent to 3.27 gears with the stock tires

Or just drive in 4th (1:1 ratio) and you'll be at 2200rpm at 70mph... slow down a bit to around 65mph to be right near 2000rpm to be on the torque peak of the I6.

A manual has no slip once in gear, so don't worry about doing this. Someone over on FTW did this and picked up nearly 4mpg...