Gearbox alternatives for Mk IV and Mk V

Is there a suitable alternative, more modern gearbox for the Mk IV 1.5, and Mk IV/V 2.5 and 3.5? Preferably a 5 speed (overdrive) and with a speedo cable drive. I guess this limits it to something from the pre 1990s era.

Quite a few people have installed the Ford Sierra 5 speed box.


The crossmember at the rear of the gearbox needs some pretty major surgery to fit the type 9 gearbox. Denis Foxely on here has done it.


What about a Jaguar 4-speed with OD box?

The OD won’t clear the cross member and it needs a new gear box cover because the ODbox is higher, So perhaps a bit less leg room.
If it is just less revs you want , the 3,77 CW&P from an OD XK120 withe the Salisbury 2 HA back end, will drop straight in. No mods required.Just change the Crown wheel and the pinion. And it’s not a ratio XK owners are looking for.

I have fitted alternative boxes to both my cars. The 1939 3.5 saloon has a Jaguar all-synchro with compact type “A” overdrive, from an XJ6. The SS/Mark 4 bell housing bolts directly on to this so no conversion plate needed. The overall length is not much more than the Moss box so the original prop shaft can be used. The od is deeper at the rear so a notch has to be cut out of the crossmember and a different mounting used. The gear lever is further back than the original so the cover has to be modified.

The 1948 3.5 Jdc has a Tremec T5 box, with a conversion kit made to fit it to XKs. This needs a new prop shaft and also a bite taken out of the crossmember, but the gear lever comes out at the same place, although it is higher.

Both work very well, but I think I prefer the Jaguar all-synchro box.

I think the Ford Sierra type 9 box does not require any cutting of the chassis, just a new prop shaft and a conversion plate at the front. The 5th gear ratio of .82:1 is not as high as the Jaguar overdrive or the Tremec.

David Davenport has fitted many cars with the Ford box.

Denis. Foxley

Hello Ed,
Would the same 3.77 crown wheel and pinion fit the ENV version in a later 1951 MKV?

Tim , umm… No. I don’t know if there are any thger ratios in the ENVs { Hypoid ENVs that is.] Remember that loger ratios with drop the revs on highways but adversely affect accenleration.
People put forward what I feel are spurius reasons to change. which are…
1 Better fuel consumption. Even if true which may ormay not be so, the cost of fuel when owning is th e elast of ones problems and probbaly not as much as the cost of conversion.

Less wear on the engine… Modern oils and fuel mean that motor car engines rarely wear out. i.e , a well restored SS Jaguar engine will out see its owner.

The engine sounds more 'relaxed ’ when cruising., there is so much ind noise and tyre noise that engine noise is lost in the cacophony. I have an Auscrank crankshaft and at 100kph engaging the clutch and not [ ie isolating engine noise] shows no difference in noise.
In a comparison of different years at Geelong Sprints, the chnging from 4 .27 to 3.77 diff added about 1/2 second to the 1/4 mile.

1 Like

Hi Ed,

Thanks for the message. I’m not concerned about fuel consumption. I’m also not terribly concerned about acceleration but would rather have lower rev’s and longer legs on the highway. 3.77:1 was the diff ratio in MK2’s and in my MK2 without engaging the overdrive it sits on about 3’000rpm at 110km. I’m guessing that the MKIV with an XK120 3.77:1 crown wheel & pinion would be much the same? With the standard ratio it would be 100km at 3’000rpm. Not that much in it really but perhaps just enough?



Timothy Fox

I fitted a T-5 from a Ford Mustang to my 48 3.5 liter saloon. Required making a 3/4" aluminum spacer plate. Used Ford clutch disk. Hardest parts was getting electric speedo output to work with Jag mechanical speedometer.

Blue skies and green lights
Jim Bolinger

Thank you all for your descriptions. I’ll ponder these in time to see if I need to go to this extent of work. I was offered a Mk VII box with O/D but I was not knowledgable enough at the time to gamble with this. Being an early O/D, it was probably bulkier than subsequent models and would probably need extra length and width in the cover panel, and maybe the chassis mount.

Yes, a Mark VII OD box would not have a tailcase mounting platform.

It’s not just acceleration from rest but through the gears on hills and passing. Too long a ratio and you may be like a man rowing a boat with the gear lever. One Register member had that problem.
When Jaguar fitted OD boxes , they al o matched them with shorter diff ratios. So e.g a MK VII with 4 speed had 4.27, the OD car had 4.55
I recall some Register members having trouble with Toyota 5 speed boxes. one who came to sell his car, explained when the prospective buyer asked about one gear not working said “well it still has 4 speeds” Which made me wonder why he’d been to all the trouble of fitting it.

1 Like