All about Diff Ratios 4.27, 4.88, 5.38 etc etc etc
This thing is quite confusing for me as well, so I thought that I might as well share some information that I am still trying to understand, for the benefit of all.
I would at present just tabulate the info as and when I get my hands on it and later on I might put it properly.
Most of the info is from teambhp and I would also be posting the relevant link along with the information I update
Also to note is that I would be changing, editing the info on this page from time to time. Therefore overtime the information on this page might change
The difs can be 5.38, 4.88, 4.22, etc
5.38 gives max torque and lower speed.
4.27 gives less torque and more speed comparatively.
Now, your 4.88 is a perfect compromise for speed and torque. However, this suitable for the OE 215/75/15 tyres which are approx 29.5″ in diameter.
Thus, for every rotation of your tyre, your axle moves 4.88 times. Or you can say that your axle needs to rotate 4.88 times to move your tyre by one revolution.
Your 31″ tyres now will cover more ground per revolution. Thus you will get more on-road speed. But, when you come to off road terrain, you will loose power since they axle has to work more to rotate the tyre and also the 31″ tyre is very heavy.
Thus, to come back to the same efficiency as the original 4.88, when you put 31″ tyres, you have to change your dif to 5.38.[/quote]
So as per the above I have checked my CJ3B and the diff ratio is 5.38 or 44:8
4.27 is 47/11 and 5.38 is 43/8.
MM540 came with 5.38:1. MM540 / MM550 with XD3P engine came with 4.88:1. Even today, Bolero SLX 4 wheel drive comes with 4.88:1 only.
Let Consider the total Toque Developed including the gearing at each wheel
For KMT-90-4WD & 4.27
(3.986 X 2.46 X 4.27 X 15.5kgm)/4 (no. of Wheels)= 162.2 Kgm per Wheel.
For KMT90-4WD & 5.38 = 204Kgm per Wheel.
The Advantage of 5.38:1 is
1) Better torque on the wheels
2) Better Crawl Speed (Slower the better)
3) compensates the Wheel Speed for taller Tyre (31″)
New Tyre OD / Old Tyre OD X Diff-Ratio = Corrected Diff-Ratio
31/28(standard for JEEPs) X 4.88 (standard for XD3P) = 5.40:1
Any thing Lower you will Lower End grunt end and gain Top End speed.