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3.9 diffs


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Hi everyone.

As part of the 110 axle swap on my Tdi powered 109 (standard transmission with Roverdrive), I'm considering regearing the diffs. I won't use the 4.74s from the existing axles because the vehicle is significantly under geared, and I am concerned about retaining the "new" axles' 3.54s as I think they will be over geared (based on experience of uphill slopes on motorways with a full roof rack with the overdrive engaged - 3.54s are taller in crease than overdrive).

I think 3.9:1 would be the perfect gearing for my application, and I know that the Rover P4 used this ratio in its Rover diff, as did the MGB GT in its Salisbury diff. The question is, will their crown wheels and pinions fit the LR carriers and diff casing?

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I Could confirm this for you tommorow, the salisbury should fit, but not sure about the p4.

You probably don't want the spendometer to go up but we do 3.8's that will fit.

Steve

Thanks. Steve. I saw the 3.8s on your website, but I have the problem of finding a matching set for the Salisbury rear axle. 3.9 would be a better ratio, but if I can't find it, 3.8 is close enough. Can you recommend anyone for the Salisbury?

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We keep salisbury 3.8's in stock ;)

I saw that later last night, and it does open the 3.8 option. The trouble is the combined cost, not only of the gear sets but also all the bits that would be needed to fit them, like the bolts, shims, seals and so on. It's a good option, though, and i like the idea of the reverse cut front diff gears. The worry about their relative cost and availability for a "down route" breakage would be mitigated by their increased strength - the chances of breaking them seem very small with the rest of the axles kept standard, including tyre size.

Given the cost, I think I'll try the existing 3.54s out first. For those long steep hills or hills with headwinds where I currently have to disengage the OD in 4th and 4.71, I'll have to use 3rd+OD on the 3.54s, and though that'd be lower than the current 4th, it may suffice. Hopefully, performance on the 3.54s without OD engages will compare closely to the 4.71 with OD despite being 5% higher, given the energy losses through an engaged OD. The £800+ cost of fitting the 3.8s will require trying the existing 3.54 gears first.

Thanks for your help, Steve.

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