R25 final drive ratio
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  1. #1
    1000+ Posts Haakon's Avatar
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    Default R25 final drive ratio

    Whos got an R25 manual handy? Need to know the final drive ratio for the MJ3 auto box. Mine is slowly starting at long last to expire and is getting a bit slippy... Oh well, been a long time coming - cant complain, as 255K km is a good run I think.

    But, I have a nice MJ3 from an R21 to drop in it, and am wondering if I should go to the bother of swapping over the diff (ie swap over the R21 rear case with the good bits in it to the R25 final drive assy). I am fairly sure the R21 unit will have a higher final drive (smaller tyres) which should be good for a couple of hundered RPM less on the highway in the R25. But if its too much higher, I will lose a lot of accelration - not an R25 highpoint to start with....

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  2. #2
    1000+ Posts Haakon's Avatar
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    anyone? Dont make me go out and buy a manual
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  3. #3
    Fellow Frogger! Ren25's Avatar
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    There's nothing in Haynes on the subject (pretty typical!)
    Cheers
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  4. #4
    1000+ Posts Haakon's Avatar
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    Bugger.. Thanks for looking though
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  5. #5
    1000+ Posts Europa's Avatar
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    Can you calculate from the info in the attachment?
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  6. #6
    1000+ Posts HONG KONG PUGGY's Avatar
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    Quote Originally Posted by Ren25
    There's nothing in Haynes on the subject (pretty typical!)
    Cheers
    Mike
    OT here, but that's the way the Haynes manuals have gone. I have three older style ones here, R17, R10 and R20 and I am sure all of them give gearbox fabrication numbers, suffixes and ratios according to each suffix. In the R17 & R10 manuals it gives a good breakdown of what ratios in what box etc, the R20 book is getting light on and the one that I had for the 1986 S**b was woeful in information. Over the years they have become just a glorified oil change hanbook IMHO.
    I'd do teh swap Hakon, as long as the R21 box is a good deal lower miles.
    MJ3= 3sp I assume...I didn't know there were r21's with 3sp auto, I had a feeling they were all 4sp. A 4sp auto would be the go and swap the final drive to an R25!
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  7. #7
    1000+ Posts Haakon's Avatar
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    R21 box is the same MJ3 3 sp auto as my R25, and is in perfect condition with very low miles.

    R21 phase 1 is MJ3, phase 2 Marseille is the AR4 4sp auto. Very differnet boxes and a swap would be tricky, as the AR4 calls for unequal length shafts, as well as vastly different electronics and computer... Later R25 had these as well.

    If I had the cash (and could justify it...) I would get a phase 2 R25 AR4 from the UK - they really need the extra cog!!
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  8. #8
    Tadpole
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    Icon7

    Hi
    i have just looked up the Renault Manual M R 249 and the ratio for the B29e MJ3 Suffix 010 is 9/32
    Hope this helps

  9. #9
    1000+ Posts Haakon's Avatar
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    Quote Originally Posted by MARK C25
    Hi
    i have just looked up the Renault Manual M R 249 and the ratio for the B29e MJ3 Suffix 010 is 9/32
    Hope this helps
    Cheers Does it say if the B29B (mine) is the same? Should be...
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  10. #10
    Tadpole
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    This manual only lists to 84, but I will see if one of my other manuals have the details.
    You do realise that there is no sector wheel for the electronic speedo on the hemispere of the R21 diff.
    Mark

  11. #11
    1000+ Posts Haakon's Avatar
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    Quote Originally Posted by MARK C25
    You do realise that there is no sector wheel for the electronic speedo on the hemispere of the R21 diff.
    Mark
    Yep, and I will also have to drill a hole in the casing for the pickup and plug the cable speedo drive hole. The sector wheel will involve pulling the diff out of both, and swapping the R25 sector wheel onto the R21 crownwheel.

    Worth doing if the R21 final drive is only a little higher. It gets most of its use on the freeway/highway, and a couple if hundered less rpm at 100 kph will useful. But dont want to lose too much zoom off the line
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  12. #12
    Tadpole
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    Hi, Well I found my techical notes on the B29B & L48K The teck stats are as follows T2/88 B29B MJ3-012 T4/88 L48K MJ3-801
    Ratios 1st 2.5 2.5 2nd 1.51 1.51 3rd 1.0 1.0 Rev 2.0 2.0 Final drive 4.1 4.1
    Sorry for the bad news, but explains why the early cars are said to have more punch, witha 3.55 diff ratio.

  13. #13
    1000+ Posts Europa's Avatar
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    Isn't 4.1 a lower ratio than 3.55, thereby meaning a 4.1 equipped car will have better acceleration but higher revs when cruising? (ie. a 3.55 car should have less "punch")
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  14. #14
    farmerdave
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    From my understanding of Mark C25's last post the ratios are the same?, ie B29B and L48K models have the same 4.1:1 ratio, not the 3.55:1 of the B29E. There's plenty of B29E gearboxes about that you could get the 3.55 ratio from, but maybe it is not the best with the unleaded motor.

    Why not rebuild the R25 MJ3? It's the same amount of work without mucking about with the final drive bearings, casings and speedo pickup.


    Farmerdave

  15. #15
    1000+ Posts Haakon's Avatar
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    Could rebuild the R25 box ( I have the factory MJ3 manual which is gold!), but I have a perfect MJ3 sitting around, and I am in tightarse mode at the moment (is very poor...), so I will use that one first.

    Well if the L48K and B29B final drives are the same, I will just swap the R21 rear case ontot he R25 final drive assy and avoid stuffing about with speedo sector wheels etc

    I just thought the R21 would be higher - it sits on similar revs at given speeds as the R25, but has smaller tyres

    Interesting the B29E and B29B have different ratios....
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  16. #16
    1000+ Posts Haakon's Avatar
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    Ooh, just read that again and understood it B29E has the higher final drive, and B29B and L48K has the lower. It just so happens I have a B29E MJ3 lying about. Excellent...
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  17. #17
    1000+ Posts Haakon's Avatar
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    Quote Originally Posted by MARK C25
    Hi, Well I found my techical notes on the B29B & L48K The teck stats are as follows T2/88 B29B MJ3-012 T4/88 L48K MJ3-801
    Ratios 1st 2.5 2.5 2nd 1.51 1.51 3rd 1.0 1.0 Rev 2.0 2.0 Final drive 4.1 4.1
    Sorry for the bad news, but explains why the early cars are said to have more punch, witha 3.55 diff ratio.
    What I actually meant to say was thanks! Good info. But I am curious - what is this tech manual you have? Sounds useful !
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  18. #18
    Tadpole
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    Icon7 R25 final drive

    Quote Originally Posted by Haakon
    What I actually meant to say was thanks! Good info. But I am curious - what is this tech manual you have? Sounds useful !
    Hakkon,

    The 3.55 would be your best option.
    You could also change step down gears:
    25/22 =0.88 or 3000K @100km/h
    26/20=O.76 [email protected]/h
    26/21=0.80 [email protected]/h
    25/23=0.92 or 26/25 not so good
    above data using 195/60/14 tires

    Al.

  19. #19
    Tadpole
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    They are dealer technical data sheets. From when i was a service/parts manager for a dealer of which i managed to retain.

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