In order to get a nominal idea of the effect of my new diff ratios on my engine rpm, I have calculated the engine rpm of my Zuk for the respective aftermarket diff ratios in the attached excel spreadsheet. It looks like I am in big trouble with 5.83s on the road (slower than Hercules) but I will have to try it out and see.

You may note from the spread sheet that the OEM stock Samurai tyre (205/70R15 which is 26.3 inch diam and 3.91 diffs) has almost the same RPM at 75mph as your 5.38 diffs if you had 35 inch tyres. Now if you changed from nominal 33s to nominal 35s (give or take manufacturers differences in tyre heights and diameters and also the squatting effect of the tyre when loaded by the vehicle weight) then your engine RPM at 75 mph, would change from 4687 rpm to 4419 rpm which would be pretty close to the stock/OEM 4273 rpm (this is assuming a samurai gearbox and transfer case).

Best Regards.....Diff Lock David. 

P.S. – To convert from a nominal P-Metric or Metric Tyre size to a nominal Inch Height or Diameter you proceed as follows:

For example take a 255 X 70-R15 tyre what does it mean? Well the R means radial and the 70 is the aspect ratio in percent. It means that the inflated height of the vertical side wall is 70 percent as high as the tyre is wide. The 255 is the width in milli-metres and oddly the number 15 is the rim diameter size in inches (note that 25.4 mm = 1.0 inch).

So taking in to account the height of not one but two side walls, here’s how you go: (255 X 0.70 / 25.4 X 2) + 15 = 29 Inch Nominal Tyre Diameter/Height

Now some light truck tyres are a 75 aspect ratio instead of 70 so using the same approach, for a 255X75-R15 tyre the Nominal Diameter/Height would = 30 Inches.

Calculated Engine RPM versus Speed in 5th-OD with nominal 35 Inch Tyres and Low Differential Gearing
	Crawl Ratios For Jimny versus Samurai Transfer Cases (1.596 / 2.716 vs 1.320 /2.123) with Samurai Gearbox
Diff Ratios	2.716  4X4 Low   Jimny Transfer Case Crawl Ratio	1.596 4X4 High Jimny Transfer Case Crawl ratio	2.123  4 Low Samurai Transfer Case Crawl Ratio	Crusing RPM with 1.596  2WD + 4 High  Jimny Xfer Case (0.864 5th at 75mph)	Crusing RPM with 1.320  2WD + 4 High  Samurai Xfer Case (0.864 5th at 75mph)	Crusing RPM with 1.596  2WD + 4 High  Jimny Xfer Case (0.864 5th at 65mph)	Crusing RPM with 1.320  2WD + 4 High  Samurai Xfer Case (0.864 5th at 65mph)
3.91 (5th)				3882.67	3211.23	3364.98	2783.07
3.91 (1st)	38.77	22.78	30.31
3.91 (2nd)	20.67	12.15	16.16
4.57 (5th)				4539.10	3754.15	3933.89	2783.07
4.57 (1st)	45.33	26.64	35.43
4.57 (2nd)	24.17	14.20	18.89
5.38 (5th)				5343.63	4419.54	4631.14	3830.27
5.38 (1st)	53.36	31.36	41.71
5.38 (2nd)	28.45	16.72	22.24
5.83 (5th)				5790.59	4789.21	5018.51	4150.64
5.83 (1st)	57.83	33.98	45.20
5.83 (2nd)	30.83	18.12	24.10
Samurai Gear Box Ratios
First	3.652
Second	1.947
Third	1.423
Fourth	1.000
Fifth-OD	0.864
Reverse	3.466
Xfer case Ratios	Jimny Xfer ratios	Samurai Xfer ratios		Road Speed	Crusing RPM at Varying Speeds with OEM Diff & Tyres	35 Inch Wheel RPM	OEM 26.3 Inch Tyre
High	1.596	1.320		80mph / 128.8 Kmph	4558.40	768.31	1022.46
Low	2.716	2.123		75mph / 120.7 Kmph	4273.50	720.29	958.56
				65 mph / 104.6 Kmph	3703.70	624.25	830.75
Diff Ratios				60mph / 96.6 Kmph	3418.80	576.23	766.85
OEM	3.909			50 mph / 80.4 Kmph	2849.00	480.19	639.04
After Mkt	4.570			45mph /  72.4 Kmph	2564.10	432.17	575.14
After Mkt	5.380
After Mkt	5.830

Here are some good links for members upgrading: