4x4 gear ratios

[barry badger]

I plan to overhaul the gearbox in my 4x4 soonish, whilst i'm at it i would like to alter the gears to bring the revs down, more for noise than fuel consumption, as it's doing over 5,000rpm at 90mph.
Does anyone know which gears from which box i can use?
Do i just change 5th or can other gears be changed too?
What do i do about the speedometer as it's driven from the transfer box?

Any advice will be much appreciated.

[Jaba]

as it's doing over 5,000rpm at 90mph.
Does anyone know which gears from which box i can use?
Do i just change 5th or can other gears be changed too?
What do i do about the speedometer as it's driven from the transfer box?

Any advice will be much appreciated.

That seems incredibly low geared.Most BXs are geared at least 21mph per 1000rpm. Yours is around 17.5 if the speedo and rev counter are accurate.
I dont have any tables for gear ratios on the 4x4.
So, assuming the gearbox is a BX standard box, if nobody else can help with the actual ratios in your box then you have two options :
Take off the fifth gears and count the number of teeth on each cog. Something like 37/28.Then it can be calculated what you need ....OR
Just fit the TD fifth gear (38/25) and see what difference that makes as only the 8v GTi gearbox needs a fifth gearset from a different model. If its too high then another BX model gearset will be required.

The speedo drive comes off the diff so is independent of the 5th gearing.

Something to think about especially as it is well worth doing and requires little time or effort.

[barry badger]

The speedo is accurate if the road sign nearby that flashes my speed up is anything to go by, when my tyres were new my speedo indicated exactly the same as the sign but now i've done about 5,000 miles i notice my speedo reads about 2mph faster than the sign.
I have several spare gearboxes, two td's and two 1.9 na's. Which 5th gear would be the best and is it easy to change. I work on cars every day but shy away from stripping gearboxes.
The gearbox needs rebuilding anyway because the input shaft bearings are slightly noisy but if I can work out the gearing first I can rebuild it later on.

Of course the speedo won't be affected by this, I wasn't thinking.
One less problem.
As an after thought I have a spare 4x4 gearbox, i'll count the teeth on fifth gear as a starting point just in case the problem is a very low diff ratio.

[mat_fenwick]

The specs for a 1991 4x4 estate are 20 mph per 1000 rpm in 5th, so something seems amiss...

But all this has got me thinking - I once bought the gearbox & transfer box, and had the chance of the complete running gear from a 405 Mi16x4. I was thinking of fitting it to my TD estate, but the main reason for not doing so (or at least getting the bits and intending to fit them) was the fact the 16 valve gearing would be too low for a turbo diesel.

If the complete gearset could theoretically be swapped over, then it removes that little problem Leaving only the transfer box/turbo issue, and the fact there's not enough hours in the day!

[BX Bandit]

oops

[mat_fenwick]

/\ Having a slow morning Neil?

I've got a ZX 1:3.66 final drive on my BX, which would further drop the revs to 3820 at your 90mph, i.e 28mph/1000 rpm.

Actually, that's really useful. Since the engine swap I've thought that mine could pull a higher gear as it will red line comfortably, and would be useful to drop the revs a bit on motorways. But I thought the TD gearing was the highest available for the BE3. Given that I've coped for the last 60k+ miles I'd have to admit it's not a burning issue, but something to consider next time the 'box has to come out...

[BX Bandit]

Or look for a 306 or 307 box mate (I think 307 anyway). It's actually a bit too big for the 1.7 but it does make driving a lot quieter!

[barry badger]

I've clocked my speed/revs from 30mph up to 90mph and they are as follows:-

30mph = 1700rpm 38mph = 2000rpm
40mph = 2100rpm 54mph = 3000rpm
50mph = 2700rpm 72mph = 4000rpm
60mph = 3300rpm 90mph = 5000rpm
70mph = 3900rpm
80mph = 4400rpm
90mph = 5000rpm

Obviously these are + or - 100rpm but proves that the gearing is very low, next step is to count the teeth on the 5th gears.

[ghaddon1701]

Sounds about right for my GTi in 4th! Definitely closer than fifth anyway.

[MULLEY]

My speedo is 10% fast, so when i'm doing 3000rpm in my N/A TXD the speedo reads 70mph, SatNav say's 63mph, i'd believe the SatNav any day of the weak over a 20 year old french plastic piece of rubbish, just my opinion of course I even drive below 30mph & there is one of those stupid signs that flashes up the 30mph limit saying to slow down, its clearly wrong as i'm not even doing 30

If you haven't got a SatNav, try borrowing one to see what that shows.

[barry badger]

If my speedo is 10% fast for example that will make things even worse, that will mean i'm doing 5000rpm at 81mph.

[MULLEY]

Your's could possibly be 10% slow?? See what a SatNav say's.

[barry badger]

I've finally found time to look into changing fifth gear, the excessive fuel consmuption has prompted this as this is my only car at the moment.
The txd is off the road due to the water pump failing but i'm going to swap the engine because it's never been a nice engine, even though the mileage is low.
Anyway back to gear ratios, i've counted the teeth on fifth and they are 37/28 so the next step is to fit a fifth gear set from a td and see what i get.

[David]

Keep us updated on the results. The gearing on the 4x4 (in mine anyway) is the same as a gearbox I got out of a 1.7D (non turbo). Although different 4x4 models might have had different gears. I was going to swap all the shafts & gears in mine with the diesel box until I realised they were the same, so I kept the original 4x4 ones as they had done less miles.

[barry badger]

I've just been trawling the net to try and find my final drive ratio, it's listed as 1:4.429.