3:45 or 3.08 Differential

[AustinM]

Hi There,

I am looking to buy a Stag this Spring and one of the main journeys would be to my house in the south of France; so long motorway cruising.

What affect does a 3:45 diff make? What's the best way to adjust the speedo?

Also, has anyone out there rebuilt a diff using the TR7/8 Crown Wheel and Pinion giving a ratio of 3:08?

Cheers

Austin

[GDPR]

With the 4 speed ZF auto and 3.45 diff mine does 30mph/1000 rpm - I have used the speedo from a Triumph 2500s that uses the 3.45 and the same wheels so error is negligible.

Haven't heard of anyone using TR7 bits and don't know if the bits would fit the Stag case..

Russ

[Steve JP]

I have the 3.45 diff in my auto, never bothered with changing the speedo, I know it always over reads by about 8mph. I have toured extensively on the continent and whenever I need to just use the sat nav showing Kmh. In the Uk it's not a problem, just make sure your speedo shows about 25 in a 30 etc

[Rubce]

I have the 3.45 diff in my auto, never bothered with changing the speedo, I know it always over reads by about 8mph. I have toured extensively on the continent and whenever I need to just use the sat nav showing Kmh. In the Uk it's not a problem, just make sure your speedo shows about 25 in a 30 etc

Hi Steve

I think you meant to say the speedo under reads. As you say, when the speedo is showing 25 you are actually doing about 30 or least that is how mine works

Regards

Bruce

[DJT]

Welcome. An Austin on a Triumph forum, that's a first

As another long-term user of a 3.45:1 diff, I can concur with the above. If you are looking at a manual/overdrive (MOD) Stag, then some have complained that it spoils the traffic light acceleration, but I didn't notice that on my old MOD Stag. If you have an auto it is a worthwhile change.

3.08:1 would be much too high for a MOD, but would certainly cut RPM at speed.

[Steve JP]

Hi Steve

I think you meant to say the speedo under reads. As you say, when the speedo is showing 25 you are actually doing about 30 or least that is how mine works

Regards

Bruce

oops - yes you're right Bruce - no excuse, just a senior moment!

Thanks

Steve

[Ian F]

Austin
would agree with DJT, I have a 3.45 TR6 diff fitted to my MOD, never found I miss any acceleration but then I don't go in for traffic light Grand Prix! My speedo under read by 10% until I fitted 15" wheels and it's now about 7% under. Oh and it will do 30mpg on a steady run @ 60 - 70 mph, so no complaints.

ian F

[AustinM]

Hi Everyone,

Thanks for the welcome; and for the advise. Keeping my wife on-board, I will go for an Auto box(her choice) and the modification of the diff to a 3.45:1 would appear to be a good upgrade.

A couple of questions:

Is there any recommended sources to acquire a second diff; one that is a 3.45:1 or one I could rebuild or should I just look to refurb the original as 3.45:1 through Rimmer Bros?

Also, what is the cost and how difficult would it be to carry out a ZF 4 speed auto box?

Cheers

Austin

[DJT]

Hi Everyone,

Thanks for the welcome; and for the advise. Keeping my wife on-board, I will go for an Auto box(her choice) and the modification of the diff to a 3.45:1 would appear to be a good upgrade.

A couple of questions:

Is there any recommended sources to acquire a second diff; one that is a 3.45:1 or one I could rebuild or should I just look to refurb the original as 3.45:1 through Rimmer Bros?

Also, what is the cost and how difficult would it be to carry out a ZF 4 speed auto box?

Cheers

Austin

The 3.45:1 diff comes from the Triumph 2000 saloon. But there is more than one casting, some of which don't marry up with the Stag mounting bracket, which is required.

The ZF conversion can be done as a 'turnkey installation' by SOC Treasurer Clive Tate, whose contact details are inside the front cover of the magazine. Costs around £3k. However several have been done on a DIY basis and at least two of us are doing it ourselves now. But it isn't a straightforward swap.

[Mike Stevens]

The 3.45:1 diff comes from the Triumph 2000 saloon. But there is more than one casting, some of which don't marry up with the Stag mounting bracket, which is required.

Actually from a 2.5PI or 2500 saloon or estate. The diff casings will fit OK, it's just the back plate that might be different. An estate diff will fit straight to the mounting plate as that is how it fits in both. A saloon diff needs an estate diff back plate fitting as the saloon mounting is different.

Sadly, a Stag back plate doesn't fit (easily) as 2 of it's fixing holes to the diff itself are in different places than those on a saloon. Don't you just love Triumph?

Cheers,
Mike.

[AustinM]

Thanks for this advise.

If the 2.5 PI or 2500 saloon or estate have the same casting but a different back plate, is it possible to use the original differential and just rebuild it with an 3.45:1 crown wheel and pinion?

Cheers

Austin

[Westfaling]

Austin
Yes that's what I did. With a 3.45 and 195/70 14 tyres my speedo is bang on. More than can be said for my modern rice burner.
Useful website here.

http://www.mintylamb.co.uk/

[AustinM]

Oops! Didn't see last post had gone through.

[DJT]

Not cheap, but reconditioned diffs of either ratio are available here:

http://www.monarch-stags.co.uk/transmission

No experience of their service, so unable to recommend or otherwise.

[flying farmer]

I did once wonder about fitting a 3.08 diff to the Triumph casing but unfortunately the crownwheel is physically too thick to fit inside the casing. This was on a dolly sprint axle when I was running the 3.9 V8, but the internal bits are the same as the saloon and the TRs.

Neil