Land Rover Gearing & Gearing Ratio Calculator

The Problem

'Modifying' and 'Upgrading' is something most of us do to our Land Rovers - but some modifications bring unwanted complex side effects & problems.

One of these is bigger tyres. Bigger means you affect your vehicles gearing, and sadly for the worse.

The bigger the change, the worse the issue - 33's 34's 35's or bigger, will be so severe that 5th gear cruising is hopeless = 4th needed - often just to keep up the same speed !.

That's 'Over Gearing', which effects both High & Low ranges.

People say 'Change the Transfer Box', that ONLY might correct 'High Range', 'Low' remains wrong as the Ratio on every LT230 TBox is exactly the same.

'To better understand 'Over Gearing' - Watch this 'Before' then rather transformed 'After' Video'

Sorting out your gearing properly (and also benefiting from a Heavy Duty CW&P) means changing CW&P Ratios on both Differentials. You may notice that we often have 'almost as new' CW&P sets of various Ratios. That's because people have realised they need to change their 'Ratios', but have got it horribly wrong! That's a hugely expensive mistake, and one that should be avoided !

The Solution !

This page is designed to give understanding and knowledge to owners as to which 'Ratio' they really need and understand, and seeing the results through the calculator program we have designed, (which you can download for free and which runs on Microsoft Excel 2003 or higher.) We have also made things easier to save you hunting the internet for all sorts of Ratios (ie All the common LR Gearbox and TBox Ratios) by including them in the bottom part of the program

(Click on Image to Download our 'Ratio Calculator' Program)

However before you start here are the basics as to how to use it, and get the best out of it !

We think the best way to understand CW&P and Ratios and the various effects is to compare what you have NOW with a 100% "Factory Fresh unmodified Land Rover". Once you have this then adding your 4x4s changes to the program and re running shows clearly the effects and problems caused.

'This video is all about explaining the differences between 'Ratios' & 'Standard vs Heavy Duty' CWPs'

Once you have these 2 extremes in front of you the Calculator allows you unlimited playing with changes - Often via CW&Ps ratio changes getting its performance closer back to 'Factory'.

You need the following information, much we have supplied in the program datasheets at the base:

• Engine MAXIMUM RPM (i.e. flat out) and maybe a chosen Target RPM for cruising at 'X' Mph
• Gearbox Type + Ratios for all the Gears (In Program )
• Transfer Case Ratios (Check the number on the casing to be sure - In Program)
• Differential Ratio (Standard then its 3.54, and Series LR 4.7 - Options in Program)
• Tyre Size when Factory fresh - Size i.e. '265 85 16' or as its measured '28' inches
• Tyre diameter NOW
• Any other changes from 'Factory' Fresh on the above.

Be 100% sure of your TBox Ratio - Do a double check !. The Serial Number is located either on the bottom left rear face of the main casing, or bottom left hand side. See the 2 pictures on the right.
Ratios are : 14D 26D = 1.003:1, 15D 27D 29D = 1.192:1, 36D 37D 38D 40D 41D 59D 61D 62D 68D 69D 70D 80D 81D = 1.211:1, 28D 32D = 1.222:1, 13D 22D 25D 43D 57D = 1.410, & 12D 20D 21D 47D= 1.667:1.

So, to generate your 4x4 'Factory Fresh' Sheet Open program.

Then under the Red Headings (Top & Bottom Left) ensure all Blue box values to match 'Factory' settings, that's, Engine MAXIMUM RPM (i.e. flat out) and maybe a chosen RPM, Gearbox Ratios, TBox Ratios, Differential Ratio, & Tyre Size from factory. The program runs automatically, when you have finished and printed off the results, you will have results for both High & Low Range and Speed per Gear in 1000 RPM increments, and also the factory speed flat out, plus (if entered) at your chosen RPM

Now to recalculate your 4x4 to as it is right now, changing any Blue boxes that are currently different to 'Factory Fresh'.

Normally this will be just Tyres, but could be TBox or even Differentials etc, match the screen to reflect exactly what your 4x4 is now, then print off and start to compare!

Advice

Study and compare things like the top speed in 'Top Gear' and compare the differing RPMs. Over Gearing will show a daft top speed, far far in excess of factory !

Lets use my 5.2 V8 90 as an example, when I 1st ran the program as a V8 90 with a LT85 and having a 90 V8 Tbox my top speed was a stunning !!...202 MPH, Factory top speed was around 89 mph!! 5th gear was useless, 4th not much better, and it was like a dead dog on a stick performance wise! By keeping everything the same, but upping the CW&Ps to 4.75s even with 35" tyres the top speed was then pulled down to 98 MPH. I changed the TBox ratio as well, and got it lowered further to 85 mph.

And the reason I went LOWER than 'Factory'? Weight. This is what we do, Cages, Spares, Lights, Sill bars, Roof racks, heavier Tyres, Recovery Equipment etc, so consider up to 5% maybe up to 10% top speed below 'Factory' if it's heavier. Weight - or adding it - is the difference between your cars performance now ...vs say you plus 4 adults and a full boot full of luggage..it kills it. So a few percentages under factory does help !

Just go easy, don't go too far and become 'Under Geared', going from too 'Leggy' in every gear to being the opposite. Under Geared might mean 0-60s become "Impressive", but in real terms you find the engine is screaming its naggers off, and you only doing 40 ! ....So our advice is 5% -10% lower than 'Factory Fresh' more to 0% if bias as a Road 4x4 vs. Off Road.

And we are always happy to give advice, just ring, in the meantime enjoy !