Differential ratio

[spivomatic]

Hi all, Ive been told a change of differential ratio will improve my cruising MPG. Apparently using a unit from a range rover will do the job. Anyone know from which model will fit the P5b 1973 ?Thanks

[johnwp5bcoupe]

I think you should do a bit more research they tend to fit our diffs which are 3.54:1! they use lower ratio's as far as the grey matter remembers apart from the Classic RR which is the same I think

some one with a bit more knowledge of the RR may help more.

WARWICK

[colnerov]

Hi, yes the RR is 3.54 the same as the P5B so no advantage,
Unless you have a P5 which I think has a 3.9 but I stand to
be corrected. Land Rovers by and large have 4.7 and P4 have
4.3 all are interchangeable.

[Warwick]

Colnerov has answered the question, but you've got me thinking John.
A couple of Australian companies make air-operated diff locks for Range Rovers.

I could fit a diff lock and eliminate all that embarrassing wheel spin and axle tramp when the P5B takes off from the traffic lights.

[johnwp5bcoupe]

[spivomatic]

So, what is the solution ? different gearbox - axle - or fit an overdrive unit. I wonder as this car has been around a while if there is any history of this type of modification. Searching this site unfortunately, is not very fruitful. You folks seem very knowledgeable and wonder if you know the answer to reducing engine PRM/speed ?
As I would like to use my coupe on a regular basis for motorway use, this mod is quite important to me and I'm sure, others.

[Warwick]

Spiv,

Do a search for ZF 4-speed conversion from V8 Sherpa vans.

Bear in mind that Search won't accept anything with less than 4 characters.
So it will search for auto, but not for V8, van or ZF.

If you are contemplating the manual route, a friend has done a very nice conversion using a 5-speed Toyota Supa box. Totally transforms the car.

I believe there is also a very nice cylindrical 2-speed overdrive device that is designed to fit ahead of a shortened prop shaft. But they are intended for the tyre-burning hoon brigade for the sole purpose of breaking traction easily with a big engine. Very very very expensive.

[johnwp5bcoupe]

Jul 20, 2010 22:12:53 GMT colnerov said:

Hi, yes the RR is 3.54 the same as the P5B so no advantage,
Unless you have a P5 which I think has a 3.9 but I stand to
be corrected. Land Rovers by and large have 4.7 and P4 have
4.3 all are interchangeable.

A bit more info Mk1. Mk1A anf Mk2 Auto's and models without OD are 3.9:1 whether the bolt/stud line up is the same I don't know but you would need to go the other way less than 3.54:1 to get any benefit

[johnwp5bcoupe]

Jul 21, 2010 6:22:24 GMT @spivomatic said:

So, what is the solution ? different gearbox - axle - or fit an overdrive unit. I wonder as this car has been around a while if there is any history of this type of modification. Searching this site unfortunately, is not very fruitful. You folks seem very knowledgeable and wonder if you know the answer to reducing engine PRM/speed ?
As I would like to use my coupe on a regular basis for motorway use, this mod is quite important to me and I'm sure, others.

LPG if your doing loads of miles!

[spivomatic]

Thank you all

[colnerov]

Hi, Warwick's remark about locking diffs for land rovers reminded
me that there is a company doing different ratio's for them as tall
as 2.83. Might be worth a chat.

www.kamdiffs.com/four_by_four.php

Colin

[spivomatic]

Thank you Colin, interesting site, 2.83 sounds good to me. I will get in touch and let you all know the results.

[spivomatic]

Well I tried this company today www.kamdiffs.com/four_by_four.php

and unfortunately they no longer manufacturer 2.8 ratio crown wheel and pinion sets, but I have found this useful information should any one be interested
Engine RPM = Known Speed (MPH) x diff ratio x tyre revs /60

So, larger diameter wheels it is then

[Colin McA]

Bigger tyres is an old Land Rover trick for better cruising.

You will be restricted the tyre size and also required speedo recalibrated.

If you want the look and comfort of the P5/P5B and better gear ratios just throw in a 5 speed manual box as this would be the simplest/cheapest way to do it.

[colnerov]

Food for thought!
Rover p6 has 3.54 Roverp6b has 3.08 The diff casing looks the same
so will the diff carrier be the same? possibly. one for harvey I think.

Rover SD1 has 3.08 is this the same? I know it is a salisbury 'type' axle
but even so, is it Rover's version with their own internals? You could
rebuild these CWP sets into your axle or fit the SD1 axle complete if
correct width, with spring pedestals replacing spring platforms.

Some Jaguar's have 2.83 CWP sets are these the same? Can these be
made to fit? try having a word with a transmission rebuilder they
would have a better idea whether it could be done

Colin

[spivomatic]

Thats a good idea using a jag 2.8 CWP. It may take some time but worth looking into.Thanks

[Phil Nottingham]

Not convinced about the Salisbury axles - these were used on LRs's and were totatally different as they were more heavy duty .

Was not aware that the P6 type would fit either but is feasible but not heard of it being tried which is odd as P6's are far far more plentiful

[Colin McA]

The P6 diff is totally different and I am not even sure you could build one into the P5 diff.

The P6 has the in board brakes and the half shafts are mounted via uj's, It is a much more complex set up with the De Dion tube.

Colin

www.rover-classics.co.uk/images/reference/thumbnailv8/final%20drive/gallery/index.htm

[baconsdozen]

I remember some years back a 'universal' overdrive was available that could be fitted to older cars by cutting the prop.The hot rodders fit jag rear ends to all sorts of vehicles there's probably room under a P5B but its one hell of a job.

[colnerov]

Hi, I was not suggesting fitting a P6 diff just the crown wheel and pinion.
Some research in parts books for part number comparison would be a
way of telling. I agree about a salisbury axle being a bit over the top in
a P5, but the SD1 axle is a salisbury "type" and a bit more lightly built
than a proper salisbury. Needs measuring to check for width plus with
the extended nose would stop axle tramp under firm acceleration or
take extension housing off and fit TR7 prop flange.

Colin

[enigmas]

This P5 final drive unit fitted was later fitted with a Salisbury differential (2.92:1 ratio) See last image. Mercedes Discs & Jag Mk2 Calipers.















Note:
The top image of the Final Drive Unit is the P5 housing that was used to set up the rear discs & handbrake mechanism.

The last image shows the RHS section of the salisbury unit welded to the axle tube. The weld can be seen to the left of the handbrake mechanism pivot. The salisbury 'tin hat' can also just be seen. (The diff 'pumpkin' can't be removed on a salisbury, but there is a large array of ratios available).

[colnerov]

Hi, nice set-up. Another option is the Dana axle fitted to Volvo's.
Used by hotrodder's there is a number of companies doing custom
made half shafts to any length and flanges drilled to any PCD.
Ratios are from 4.1 to 3.3. Different widths depending on 200, 700
or 900 series, rear discs, drum handbrake and some with LSD.
Apparently very strong will take tuned Rover V8s.

[Deleted]

Hi Enigmas lot of nice work there are you runing Air Shocks on your P5 ?

[enigmas]

No Kanter, the red 'bags' you see are called polyairs. They only require low pressure (unladen requires about 4 psi). A bicycle pump suffices.

[Smallfry]

Right. I have come into this a bit late but here is what I know.

A P6B differential unit (just the works) CANNOT be built into a P5 axle.

I have no idea if the crownwheel and pinion could be fitted though.

You CANNOT fit any Salisbury gears to a Rover axle because the Rover is a spiral bevel unit, and the Salisbury is a hypoid unit (look up the difference on the internet)

A 2.88 final drive is a bit high for a P5B. You will kill your already feeble acceleration, and it might struggle on steep inclines or a full load. It will almost certainly ruin your gearbox because it will cause the changes to slip more.

Bigger wheels and/or tyres will have a similar effect

The answer is an overdrive............a four speed ZF auto gearbox is the easiest and most practical, and in top gear, will more or less give you the equivalent of a 2.88 final drive. Thus retaining your acceleration and pulling power, and will knock off about 1000 RPM at motorway speeds.

There is/was a company who advertised a kit in Take Five, but it was ridiculously expensive, so I am working on a kit of parts ATM.