Rover V8, Discovery diffs, AND overdrive??

[ejparrott]

I've only come to this today, and by the looks of it I've got a magic Series.

I've been running a 200TDi now for about 7 years. In that time it's always been coupled to a Series 4-speed gearbox, non of which it's ever munched, except for a question mark hanging over the current one. The first was the box it came with and I had a go at rebuilding one, which all worked fine but was noisy on the lay-shaft bearings. I then had some money spare and as we use the 88 every day I decided we needed a good box so had one rebuilt by a company in Cov. That was always a bit reluctant to select first, and after a year started crunching down into second, now it won't select second at all, but until I strip it to find out what's gone wrong I'm not prepared to say the TDi killed it, if it did then it's the first.

So, TDi + 4-speed. I've also always run it with an OD, that was fitted when we had a 19J, During the life of the 19J I also fitted 3.54 diffs in the axles, on the ends of which have always hung 7.50's or 235/85 tyres. Driving round town at 30mph I use third and OD, always. The wife tries to use fourth but it's much too tall, so I know if she's speeding.... I drive sensibly, it was the 19J that snapped the half-shaft, we drive her in town every day. she's done quite a few off-road days, and we go out laning quite often. On our holiday runs I take the satnag to record accurate mileage every trip we make, and I brim the tank before leaving and again on return. Last trip to Wales a few weeks ago worked out at 40.08mpg. I have never in 7 years found that there was anything wrong with the gear ratios, I regularly get around 40mpg on runs out which I'm not going to worry about, and the only regret I have is that the Series didn't have a 5-speed gearbox from the factory that I could fit my overdrive too. I am now seriously thinking about converting her to a 5-speed LT77, but this has absolutely nothing to do with gear ratios or TDi's killing series boxes. I'll be doing it as an exercise for one, and because the LT is undeniably quieter than a 4-speed. It's also the box the engine is meant to run with and it doesn't leak oil everywhere.

[Snagger]

Fifth will be lower than overdrive, so will be much more usable anyway. Still a little tall overall, but should be better than the Series box.

[missingsid]

OK so I am still a bit supprised about the use of 3.54 + OD so I looked at the Ashcroft Gear Ratio Calculator and came up with this comment and resulting calculation.

The ideal target RPM is 2,700 revs at 70 mph in top gear. (from website).

Transmission setup

Gearbox:

Variant: Series 3

First Gear: 3.6:1

Second Gear: 2.22 :1

Third Gear: 1.49 :1

Fourth Gear: 1:1

Transfer Box:

Variant:Series 3

High: 1.148:1

Low: 2.35:1

Overdrive:Fairey

Ratio: 0.78:1

Diff Ratio: 4.11:1

Tyre Diameter: 31 inch - 7.50 R 16

High Table Overdrive Speed/Gear

Fourth Gear RPM

10 MPH 399

20 MPH 799

30 MPH 1198

40 MPH 1597

50 MPH 1996

60 MPH 2396

70 MPH 2795

80 MPH 3194

90 MPH 3593

100 MPH 3993

110 MPH 4392

120 MPH 4791

So this would sugest that the ideal is a set of 4.11:1 diffs and an Overdrive.

3.54 and no OD gives

70 MPH at 3086 RPM so too low geared?

and

3.54 and + OD gives

70 MPH at 2047 RPM so too high geared?

[ejparrott]

That seems to equate roughly with the 1700rpm ISH I seem to be doing at 56mph, which for me I find absolutly fine. I don't think it's too high geared, and I've been driving it like this for years.

Why should I being aiming for 2700 rpm anyway?

[discomikey]

OK so I am still a bit supprised about the use of 3.54 + OD so I looked at the Ashcroft Gear Ratio Calculator and came up with this comment and resulting calculation.

The ideal target RPM is 2,700 revs at 70 mph in top gear. (from website).

Transmission setup

Gearbox:

Variant: Series 3

First Gear: 3.6:1

Second Gear: 2.22 :1

Third Gear: 1.49 :1

Fourth Gear: 1:1

Transfer Box:

Variant:Series 3

High: 1.148:1

Low: 2.35:1

Overdrive:Fairey

Ratio: 0.78:1

Diff Ratio: 4.11:1

Tyre Diameter: 31 inch - 7.50 R 16

High Table Overdrive Speed/Gear

Fourth Gear RPM

10 MPH 399

20 MPH 799

30 MPH 1198

40 MPH 1597

50 MPH 1996

60 MPH 2396

70 MPH 2795

80 MPH 3194

90 MPH 3593

100 MPH 3993

110 MPH 4392

120 MPH 4791

So this would sugest that the ideal is a set of 4.11:1 diffs and an Overdrive.

3.54 and no OD gives

70 MPH at 3086 RPM so too low geared?

and

3.54 and + OD gives

70 MPH at 2047 RPM so too high geared?

70MPH @ 2047MPH seems perfect to me

[Gareth Dickens]

OK so I am still a bit supprised about the use of 3.54 + OD so I looked at the Ashcroft Gear Ratio Calculator and came up with this comment and resulting calculation.

The ideal target RPM is 2,700 revs at 70 mph in top gear. (from website).

Transmission setup

Gearbox:

Variant: Series 3

First Gear: 3.6:1

Second Gear: 2.22 :1

Third Gear: 1.49 :1

Fourth Gear: 1:1

Transfer Box:

Variant:Series 3

High: 1.148:1

Low: 2.35:1

Overdrive:Fairey

Ratio: 0.78:1

Diff Ratio: 4.11:1

Tyre Diameter: 31 inch - 7.50 R 16

High Table Overdrive Speed/Gear

Fourth Gear RPM

10 MPH 399

20 MPH 799

30 MPH 1198

40 MPH 1597

50 MPH 1996

60 MPH 2396

70 MPH 2795

80 MPH 3194

90 MPH 3593

100 MPH 3993

110 MPH 4392

120 MPH 4791

So this would sugest that the ideal is a set of 4.11:1 diffs and an Overdrive.

3.54 and no OD gives

70 MPH at 3086 RPM so too low geared?

and

3.54 and + OD gives

70 MPH at 2047 RPM so too high geared?

This is the similar conclusion I came to. I have a Toyota 5 speed behind a Ford 302 V8 (5l) and 33" tyres and the towing cam I selected puts the maximum torque at 2500 rpm. I found a chipped tooth on 2 of my 3 Salisbury crown wheels and so wanted to put 4.1 ratios as that would stretch the legs a bit for freeway driving. The price convinced me otherwise. I'll be fitting 35's when my 33's are knackered.

With an engine with more torque I would suggest:

1) a five speed gearbox.

2) a transfer box with 1:1 high range.

3) 4.1 diff ratios

[Scooby Jim]

What diffs are 4.1??

I will probably not fit the OverDrive until I have some 4.7 diffs, and then fit the overdrive. I'll see what it is like with the disco diffs, then I can always fit the series diffs.

[Pollywog]

Rover P4's had different ratios and are direct swaps if you can find them, I think they came in 3.9, & 4.2/4.3?

4.1 are available from ashcrofts.

I don't see the point in you waiting to fit the overdrive, you may as well fit it now and if you don't like it with the current diffs just don't use it! You may as well try it now before swapping diffs or you'll never know what it drives like.

[Scooby Jim]

Rover P4's had different ratios and are direct swaps if you can find them, I think they came in 3.9, & 4.2/4.3?

4.1 are available from ashcrofts.

I don't see the point in you waiting to fit the overdrive, you may as well fit it now and if you don't like it with the current diffs just don't use it! You may as well try it now before swapping diffs or you'll never know what it drives like.

Thats true, but would involve getting my Land Rover on the road lol.

Plus I need one or two things for the overdrive.

[discomikey]

you will most likely find yourself fitting the overdrive shortly after the diffs anyway if you do a fair bit of motorway driving

[Bowie69]

Thing with an overdrive.... you can switch it on an off

I would stick with the 3.54's as well, and get the thing on the road! Otherwise this is all conjecture