OK Gustav,
Here's the tedius technical detail:-
Standard Series Gearing works out at 1 crank rev = 0.185336 wheel revs.
Road-Speed to revs depends on wheel-size though, but maximum power is made by most Landy Engines around 3,800rpm, so, you get a typical 'cruising speed of'
97Km/h or 60mph MPH on 205R16 / 6.50X16 sized tyres, or 104Km/h or 65MPH on 235/85R16 / 7.50X15 sized tyres.
Maximum engine speed for a Landrover engine is around 4,700rpm, though they dont usually have enough power at that rpm to 'pull' the gearing they have so wont go this fast, but if they did, you would get; 120Km/h or 75mph MPH on 205's or 129Km/h or 80MPH on 235's
I suspect with your Mercedes Diesel Engine, your problem is that the engine wont rotate to the same speed as a Landy Engine, and even 3,800rpm, and you want to riase the gearing to be able to do the same sort of road speed at the sort of rpm your engine is making some useful power.... perhaps 1,800 rpm?
That is going to need a LOT more gearing than an overdrive unit alone can offer.
An Overdrive unit raises the gearing by just under 25%, so to go 100Km/h 60mph, you would knock the revs back only to 2,800rpm (approx) which may be 'enough' to help you hold a reasonable road speed, but it will probably still be 'thrashing it.
The alternative to an Over-drive is a pair of Range Rover differentials, they also give a 25% lift in gearing, but with a big, high torque, low reving diesel, those AND an Over drive might get the rpm down some-where more useful.
The other suggestion is an Ascroft 'High Ratio' transfer box 'kit' that gives a 33% raise in gearing.
Now, an over drive gives extra 'gears'.... you have the original ratios available in the gearbox, plus 'inbetweenie' ratio's and 'one above' provided by the OD.
This makes them very versatile, becouse they give you more gears and no compromises over the original gears, as you still have them too.
If you fit Range Rover differentials, then you get higher gearing in every gear... and the spaces between the gears get bigger, and you dont have anything in between.
On a big low reving mercedes diesel, though this MIGHT not be unhelpful.
But it depends on how confident you are that your engine has the power to pull gearing that tall.
An Ascroft Hi-Ratio X-fer gives you more than a set or RaRo Diffs, but again, raises the gearing accross the board, making the gaps between gears EVEN bigger.... BUT it doesn't effect the 'low-ratio' gear-set, so you still have the standard gearing there.
Now I am not entirely familiar with the Merc 300D's power charecteristics, but I believe it is NOT as powerful as more modern diesel engines, but is a 'little' more willing than older ones.
The 'big' Perkins diesels when fitted into Land-Rovers had a big problem that thier rev-limit was often little more than 3,000 rpm, I suspect your Merc motor may rev a little more, perhaps 3,500, maybe even 3,800, but I doubt it has the tractor type torque of the old Perkins low down.
That torque used to 'eat' Overdrive units, and for that reason I'm suspiciouse it would prove amazingly helpful to you. Ascroft +33% transfer box, was good with a Perkins, becouse it gave that bit more gearing, and didn't break.
I think it may be a better choice for you, though I'm not sure the Merc motor would have quite the 'stomp' of a perkins.....
However, it is more likely to be better compromise than Ra-Ro diffs AND an over drive, which together would probably be far too much, EXCEPT you dont HAVE to use the OD.....
I dont usually reccomend them, BUT if your Merc motor is suffering from too much low down power and a restrictive rev range, I'd be TEMPTED, as its a lot cheaper than an OD to suggest you try a pair of RaRo diffs, and see how it goes.....
IF it gives you enough gearing, great, if you suffer from the wide-spaced between gears and / or lack of low enough gears for off-road, then tells you you do need to go for an OD.
If it's STILL not enough, then would tell you you need OD + Diffs or an Ascroft..... but a set of RaRo Diffs are usually about 1/2 the price of an OD and so worth the try.
