Hello ,
New here , i'm having trouble with my 1990 F150 4x4 . I have a 5.0 HO bored .030 , aod trans and borg warner 1356 transfer case and I have rebuilt all 3 in the last year . They all have less than 3000 miles on them and now my rearend has went out . I figured with everything else rebuilt , the rearend couldn't handle it . I had alot of play while driving and also had a whine coming from the differential when I let off the gas and it also has a vibration coming from somewhere (maybe driveshaft). This week I had a popping noise that I thought was coming from the front , but as I came out of the woods from hunting yesterday , there was a puddle of lube leaking from the differential . You could see where something slammed into the cover from the inside and put a hole in it . I took off the cover and one of the pinion gears shattered into about 5 pieces and has been slamming into the cover occasionally . There is no tag on the cover but I believe I have the 8.8" rearend by looking at the cover shape and looking at the internals . My problem is , I don't know what ratio I have so I can replace them . How can I figure out what the ratio is ? I was thinking of putting a 4" lift and 36" tires on it soon , so would I need to go with a different ratio for the bigger tires without loosing my gas mileage ? I'm running 31" tires now and didn't know if the slightly bigger tires would make that big of a difference . I figure I would need to change the gears in the front also if I change the back . If it isn't going to make a huge difference in gas mileage I would rather just go back with stock ratio .

 

Check the axle code on the door sticker, then go here.

Ford Axle Code Chart .: Articles

 

Most likely the truck has 3.55 gears which is code H9 on the drivers door decal. 36" tires are not a little bit bigger than stock they are a LOT bigger and will drastically reduce the final drive ratio, transmission life, and gas milage. You will need to step up to 4.56 gear just to maintain stock overall drivetrain gearing. You may also need to recalibrate the speedo, this is done by replacing the driven gear on the transmission end of the drive cable, there are a range of gears availabe for this. A 20t orange gear is required for the above tire and gearing combo...

 

It is 3.55 gears , when I took it out today I seen it was stamped on the ring gear . I'll prob just keep it stock for now , thanks for the help !

 

Can you explain how you figured out the correct speedo gear?
the 87 in my sig has 31's on it and the speedo is so far off its driven me crazy