June 2007: David Landreth Smith:

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:



and the Hydoboost Brake Conversion URL:


Wilwood URL:


Power Boosters URL:



 Heavy Duty Welders

As at September 2009


CASFAB who do welding commercially: recommended by David Landreth-Smith

LS Fabrication in Charlieville or Gormandy in Sea Lots: recommended by Sham Mahabir

Dessi: Recommended by Sheldon Mohammed

KANSON's Engineering in Couva: recommended by Ryan Ramsamooj



David Landreth-Smith also noted:

After welding please spray the anchor with ZINCIT from Peakes (or other hardware) or go to the trouble of getting it hot dip galvanised in the North.


Also first test the quality of the finished weld by

1)      Visual inspection and perhaps Dye Penetrant testing. Look for good penetration on both sides of the weld (watch out for lack of fusion/cold laps which will fool you in to thinking you have a good weld when you don't).

2)      Mechanically load it and bend it (not to destruction) and hold under the stress of high load for several hours (overnight) and also shock load it several times with a 2 or 3 lb hammer very vigorously. Then use it happily once it survives your above inspections.

If you are really concerned about weld quality you can insist that it be MIG welded which is generally better than stick for good quality control. Also MIG is easier for a weld to get good results than normal Stick arc Welding (not everyone has a MIG machine though but CASFAB should). I Do Not suggest you ask for TIG unless you can find a real alloy specialty welder (generally in South and Central T&T) since TIG requires a very high level of skill and this means that if improperly done it will be far worse than stick welding or MIG welding.