Changing the Rear End Gear Ratio

Picked up a 2000 Ranger 3.73 7.5 rear end with 60,000 km on it for $150. I was going to use 4.11 but couldn't find one and the 3.73 came up. I've had the 3.73 for over a year now and suggest going to 4.11 for anyone doing this swap. The extra 10% change in gear ratio translates to a 10% change in engine rpm which would be nice for low rpm pulling power. 3.73 are good, 4.11 would be great!

There are two brake drum sizes for Ranger pickups: 9" and 10" diameter. The 10" diameter besides being bigger are also 3/4" wider - way better stopping power! They are bolt-in swappable and nothing is required to do to the master cylinder

The 3.73 gears are fantastic! What a difference, I have pulling power down to 1500 rpm - not much but at least its usable now! I've found that my shifting habits have changed, instead of shifting at 3000 rpm, I'm shifting at 2500 and driving around town in 4th at 1600 rpm with no problem. If you are going to spend money on a modification - CHANGE THE GEAR RATIO FIRST!

Identifying rear end ratios - How do you identify a rear end's ratio? There's easy two ways: the clean way and the dirty way. The clean way is to check the driver's side door for the saftery standard certification, it will have the axle ratio code listed. The dirty way is to crawl under the truck and check the differential tag which is most likely more accurate - just in case someone swapped out the rear end like I did. The Ranger Station has a great resource on listing the axle codes and reading the axle tag.

Swapping out the rear end - Why swap out the complete rear-end rather than change the gears? You need a lot of experience, tools, patience and time to properly change and setup the rear end gears. I investigated it and came across several amazing websites that describe how to do it step by step. They scared the bejeebers (now that's an old word) out of me! Here's an example website: Chris Neighbors 8.8 Gear Install

Swapping out a complete rear end is actually pretty simple and straight forward. Here's what you have to do:

  1. Jack up the truck
  2. Remove the rear wheels
  3. Disconnect the brake antilock sensor if you have one
  4. Disconnect the brake line for the brake hose at the frame. Plug the brake line to stop the fluid from leaking.
  5. Disconnect the brake hose at the frame by tapping the c clip with a hammer and punch
  6. Disconnect the lower shock bolts. You may need new shocks if the bolt has frozen in the shock. When I replace shocks, I grease the bolt shafts so that they don't freeze in place.
  7. Disconnect the parking brake cables - probably the hardest part! Compare the new rear end's parking brake cables with the old one:
    • If the ends are the same and the new ones are in good shape, remove the parking brake lines at the adjustment point under the gas filler door
    • If the ends are different (2000s are different from 1992s), remove the cable at from the back of the brake backing plate. This means you will be dismantling the complete rear brakes which is a good time to turn the drums, grease the auto adjusting parts and change the shoes if needed.
  8. Change the transmission speedo gear - unbolt the one bolt holding the cable end to the tranny and pull.
  9. Remove the speedo gear by slipping off the black plastic c clip.
  10. Disconnect the driveshaft - 4 bolts and its off. Put the truck in gear to stop the driveshaft from turning. Tap the wrench with a hammer to break the locktite. Heating might help. Tap the driveshaft to separate it from the diff if frozen on from rust.
  11. Remove the 4 bolts holding each of the axle clamps on and the rear end should drop down.

Installation is just the reverse. There are centering pins on the springs that must mate with the centering holes on the axle mounts. Make sure that they line up when remounting

Tips

  • Plan to spend a weekend - I started Sat at noon by myself and finished Sunday morning.
  • I jacked up the truck about a foot up but higher would be better. The frame in the back end of the truck is quite high and my jack stands with blocks of wood could only go up about a foot.
  • Wire brush all the bolts that need to be removed and spray them with WD40 before trying to remove them.
  • Buy brake cleaner, you may need brake shoes, silicone gasket maker and diff gear oil first before the stores close! You may not need the gasket maker and diff oil.
  • A large rubber vacuum line cap/plug will cover a brake line and stop it from leaking.
  • Try to find a rear end that's the same year as yours - see gotcha's below!
  • Ideally you would want a limited slip rear end but they are hard to find!
  • Rear ends with 10" drums are preferred as they are bigger and have wider shoes - better stopping power!
  • If you are not sure which gear ratio you should choose 3.73 or 4.11, go with the 3.73s. I have small 14" tires 195 - 70R14 (24.5" tall). If you have taller tires than you can get away with 4.11s. Here's a tire size calculator to figure out your tires dimensions. Here's a engine rpm versus mph calculator to figure out what rpm you would be in each gear. My goal was 2700 rpm in 5th at 70 mph. Wallace Racing has a whack of online automotive calculators that you can play with!
  • Calculate the new speedo gear by using this formula: new speedo gear teeth = old speedo gear teeth x new gear ratio/ old gear ratio
    • The speedo gears are color coded and are labeled with the number of teeth or you can count them
    • There's only a limited range of speedo gears. I needed 22 teeth (acutally 21.7) but the largest number is 21 which ends up to be just a 3% error.
    • Here's the Ranger Station's resource for identifying speedo gears by color and part number
    • I strongly suggest that you bring the old speedo gear to the Ford dealer (the only ones who sell speedo gears) when you go.

Gotcha's

Gotcha's are things that happened that you didn't expect.

  • There are different parking brake cables - you may have to swap out the new and old parking brake cable which requires dismantling the rear brakes
  • Some time in the late 90s, Ford changed the rim centering hub on Ranger rear axle from 2.5" dia to 2.75" dia. My rims didn't fit. :
    • Plan B: swap out the old axle shafts for the new ones. Ford changed the axle shaft lengths in the late 90s. The 92 axle shafts are 3/4" too short!
    • Plan C: I had to turn down the center hub on the axle shafts from 2.75" to 2.525" on a metal lathe. An alternative is to buy new rims but then I would have two different sized rims and need two spares. It is very easy to turn down the center hubs.
    • In 1992, you could get two drum sizes: 9" and 10". The new rear end used 10" drums but the centering hole is 2.75", so I had to buy a set of 1992 10" drums which have 2.5" dia centering hole and I ended up with a new pair of drums from HELL!

      Ever since, I changed out the rear axle, I had a strange cyclic vibration coming from the rear of the truck at highway speeds. It took 1 1/2 years to figure out that both these drums were out of balance! I never had a good feel about the size and number of balancing weights on them.

      I finally found out that the brand new drums were out of balance. This was after changing the pinion angle, both axles, balancing the tires, changing the rims, tires, replacing the new shocks and adjusting the driveshaft in all 8 positions, I replaced the drums (Aug 2007) with new ones (Nov 2008) that didn't have any weights welded on. The vibration was finally gone!

      Should of gone with my instinct when I first looked at the out of balance drums. There is no reason except for bad quality control and casting to have that many weights on a brake drum! There's 4 on one drum and 6 on the other!

    Changing the pinion angle - check to see if the pinion angle is correct, you may have to add shims to correct strange vibrations!

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Copyright March 2011 Eugene Blanchard