I found it virtually impossible to find any intelligent information on a better-than-stock gear set for my Ranger that would run 100% silent. The information I came across was always bits and pieces of old information that always left me with more questions than answers. I ended up making a decision with less than adequate information to start. In the end I couldn't be happier. Just got lucky I guess. All in all this is what prompted me to review the products I chose to hopefully help someone else with the same decisions. As for the Auburn Pro Series LS differential; this I had already had experience with so already knew I wanted another.

My Ranger (I bought it brand new) = 2002 Regular Cab, 4x2, 2.3L DOHC 4cyl, 5spd Manual and 7.5" 10 bolt rear pumpkin. It originally had 3.73:1 gears without limited slip. Current mileage is 227,300 miles.

New Ring & Pinion gears are:
Motive Gear Performance
High Performance Ring & Pinion Series
4.56:1 ratio, Part# F875456
41 tooth ring gear and 9 tooth pinion made from 8620 steel.
They include: computerized heat treatment, Gleason 5 cut technology, solid shank pinion shaft, central contact pattern, indexed tooth spacing, cutting edge lapping, and CNC start to finish.
I bought them through Summit Racing Equipment for $183.97.

New Differential:
Auburn Pro Series Limited Slip Differential
Part# 542048
IMPORTANT INFORMATION: Auburn does include a milled pinion shaft with this differential at no extra charge to allow for clearance past the fatter ring gears of 3.73-4.56 ratios. But it still did NOT clear the teeth of the Motive ring gear in the set I listed above. Therefore my mechanic did have to file just a tiny bit off two of the teeth of the ring gear for the pinion shaft to clear. No big deal but I wanted to mention it. General FYI; Auburn does include their own limited slip additive with the differential.
I bought it at Summit Racing Equipment for $565.97.

New very thorough axle/ring & pinion overhaul kit:
Ratech Deluxe Ring and Pinion Installation Kit
Part# @ Summit Racing Equipment = RAT-3006K
Note that it does NOT include the 10 new cover bolts as claimed on Summit's website, but it does include: 10 new ring gear bolts, cardboard cover gasket, pinion shims, carrier shims, crush sleeve, pinion seal, pinion nut, head bearing, tail bearing, carrier bearings, axle seals and axle bearings. All bearings were the KOYO brand from Japan.
I bought the kit at Summit Racing Equipment for $109.97.

For installation I took it to a mechanic who is as good as it gets and who I've had experience with in the past. If it matters to anyone, my mechanic said he set the pinion preload to 35 inch-pounds, and backlash ended up between .005" - .007". After it was all installed he charged me $1030.00.

At this point I have 8700 miles on the above setup. From day one the gears have run 100% quietly. No sound whatsoever, and smooth as silk. The tire size I currently run is LT235/75R15 which are 28.86" diameter (the original P225/70R15 from Ford were 27.4" diameter). I have not yet had my speedometer calibrated to the gear change so my truck is currently limited to a top speed of 78mph instead of the 91mph Ford had it limited to with the original gear set and original tire size.

I've kept track of various rpm at various mph in various gears and thought I would share that info here too. This is not based off of a calculated chart but rather the actual rpm my tachometer shows in the truck. For each transmission gear and mph the rpm will first be listed for the 3.73 ratio and then for the 4.56 ratio. Tire size is 235/75R15 for both.

3rd gear @ 55 mph = 3700 rpm * 4510 rpm
3rd gear @ 60 mph = 4100 rpm * 5000 rpm
4th gear @ 55 mph = 2500 rpm * 3025 rpm
4th gear @ 65 mph = 3000 rpm * 3510 rpm
4th gear @ 80 mph = 3510 rpm * 4360 rpm (78 mph with 4.56)
5th gear @ 55 mph = 2000 rpm * 2475 rpm
5th gear @ 65 mph = 2400 rpm * 2925 rpm
5th gear @ 80 mph = 2990 rpm * 3480 rpm (78 mph with 4.56)

Obviously my Ranger doesn't have a digital tachometer and I had to make educated guesses for many of the numbers. But for all practical purposes they are correct. BTW, when I still had the 3.73 gear set, and with the 235/75R15 tires (bigger than stock), the truck would do 96 mph before cutting out.

The reason I wanted a higher than OEM quality ring and pinion gear set is because I used to run a 5.13:1 ratio gear set that was no better than basic OEM quality (can't remember the brand name off hand) and 4 teeth ripped off the pinion gear after I took it for an unintentional hell ride in some mountains back in the middle of nowhere. The teeth didn't actually break off till I was pretty much back home a few hours later.

I didn't mean to beat it that hard but it was either get back up the hill or don't go back home. At the time I was running 31" BFG M/T tires with a different Auburn L/S Pro Series differential and was kicking rocks off that hill side way bigger than bowling *****. Had to back back down the hill 5 times before making it over the top the 6th time. The truck was jumping around like it was on a trampoline and I literally had to hole the shift lever in gear as hard as I could or it would jump out of gear constantly on my way up the hill.

It was a roadway/pathway that I was on on the hill, but it was all loose rock and gravel and seemed steeper going back up than when I first went down it hours before. Anyway, that experience was the reason I wanted a better set of gears. Plenty of people talk about stepping up to an 8.8" rear pumpkin which I don't think is a bad idea but also don't think is necessary especially if beefing up the gear set and all. One nice thing about the 8.8" pumpkins is a much greater selection of ring and pinion gear sets (more available ratios from more manufacturers).

The reason I didn't go back to a 5.13 ratio gear set was because even with the 31" tall tires at the time the rpm on the interstate highways (75 mph speed limit where I lived then) was just a few hundred rpm higher than I was really comfortable with. And it would have been even worse with the smaller tire size I run now. With the tire size I run now I'm pretty happy with the 4.56 ratio gear set, and I only get up to interstate speeds (70 mph where I live now) once in a great while.

Tire size being equal, I've lost 3 mpg with the 4.56 gear set over the 3.73 gear set. That basically equates to 22-26 mpg instead of the 25-29 mpg I used to get depending on driving conditions.

Real quick, about the Auburn differential, all I can say is I love it. Loved it in the past with 5.13 gears and love it now with 4.56 gears. Barely touch the gas pedal and both rear wheels are basically locked. Very aggressive, and exactly how I like it. Is it a perfect solution for every single scenario that exists? No. But neither is anything else and I personally love the Auburn Pro Series Limited Slip differential over all other options. To each there own and depending on what you want and what you do something else might work better for you.

Well, that's all I can think to say. Hope this helps in the decision making process for some of you. I tried to give all the information I could think to give but if you still have questions, ask, and I'll get you some answers when I can.

 

A bit of information from Motive Gear and Auburn...

Motive says;

"BREAK-IN PERIOD REQUIRED Light driving for a period of 500 miles and not at a constant road speed (50-60 MPH) for any ratios lower than 3.73. No towing for extended periods."

"Break in of gears is paramount to gear life. Initial run in should be at low speeds. This will enable gears to run in without overheating."

"Change the rear end fluid at 1000 miles. This is because during break-in, the gears and bearings will have some material chip off or wear off, contaminating the fluid. Many chemicals used in heat treating or case hardening are from the potassium nitrate family. This mixed with gear oil produces a number of corrosive acids. These acids also break down the oil film, rendering the lubricant useless. Steel to steel contact then happens leading to ultimate failure."

"Fill the unit with oil. Use recommended lube e.g. manufacturers specs. Many times a rear end is test driven without oil. The only oil present is initial lubrication on the bearings. Another practice is to assemble rear ends dry. This is a concept believed to help the rear ends run in quicker. This is NOT general practice. The gears and bearings lose their heat treat immediately, causing extensive damage. Gears should be run in at moderate speeds with maximum lubrication."


Auburn says;

"The Auburn Gear limited-slip differential design has been extensively tested with high quality non-synthetic 80W90 hypoid oils treated with GM or Ford friction additives (3 oz. of additive will treat 1 quart of oil). To avoid differential clutch chatter (noise) and for optimum performance, use the oil and additive described above. Use of other additive and oil types may cause differential clutch chatter."

"It is recommended that the axle lubricant be changed every 7500 miles or as required by your vehicle's service schedule. Lubrication breakdown can lead to accelerated wear to all rear axle components."