I pulled the truck out this morning to get the tag info off the rear differential & also off the sticker in the door jamb.
Particulars:

Date of Mfg: 04/87
Exterior: 117
Type: F141
Body: KD
Axle: H9
Spring: CL

Rear Diff Tag: S817P / 3L 55 8 8 7D06

I want to change out the ring & pinion to change the ratio so as to increase the mpg on this truck. I never need it to climb a tree - lol.
Any help as to what I have & suggestions what to change it to very much appreciated.

RickTheFix

 

Guess it might help to include truck info:
1987 F150 XLT Lariat, 4x4, 5.0L, auto trans, auto lock hubs, short-bed, standard cab, fleet-side

 

Honestly anything less than the 3.55's might lose you some mileage, but if you want to try it I have some 8.8 gears out of my Mustang. They are 3.27, and were replaced with 4.10's at 40K and are in perfect condition.

 

Another thing I might add is a lot of people have seen better city mileage with a taller gear. I know mine in my mustang went from 15 to 19-20 in town with a switch to 4.10's. If you do a lot of freeway driving however they would not be beneficial.

 

Yes H9 is 3.55 limited slip diff. Also remember that if you change out the rear gears you will also have to change the front axle to the same ratio.

 

^^

The expense of changing both ring and pinions will likely take many miles to pay for itself in fuel savings.

Are you looking to go higher (lower numerically) or lower (higher numerically) with you gears? I would think higher then 3.55 might not be good for a 5.0, as it lacks excessive low end torque.

Remember that a smaller engine like your 5.0 will reach its volumetric efficiency at a higher rpm, and will likely have better fuel efficiency with a lower gear, as WhiteBeast stated.

Granted, there is alot more to it...

 

If your tires are stock sized or close 3.55 gears is just about perfect for that truck, and if it's highway milage you want to improve it'll be far more cost effective to swap in an overdrive trans(AOD) than change gearing in both axles, the added plus being you won't lose any pulling power.