Amazon: Re-gearing an M40 gearbox

[Ron Kwas]

Ray;

It's not a solution one hears about often, but you could lower your Diff ratio ...that would give you the top end Rev reduction you are looking for (at the modest performance price of course, of off the line acceleration)!

Cheers

[Derek UK]

This Amazon is likely to have the 4.10 axle. I don't think there is a lower number diff for this axle. The later disc brake 1800 axles can be found with a 3.90 on automatics. I don't think I would want to have the raised first gear that you would get if you used this. It would be like using second as your first gear and having to slip the clutch a lot.
Using 185/80 tyres will help a bit and these have been used on Amazons. Not ideal though on 4" rims. 4 1/2" Estate rims help a bit.
Remember that the 122S and 123GT with OD use the 4.56 axle so the actual rev difference at, say, 70 mph, isn't drastically different to having the 4.10 axle. Some argue that fitting the OD when you have a 4.10 axle does nothing for performance and that is course true as the other 4 gears will be just the same. What it does give you is quieter and more relaxed cruising. MPG wont change much as the engine will have to work a bit harder pulling the higher gear. If you can resist using the OD until you get up to 45-50 you may get an extra 1-2 mpg but it's still down to your right foot.
My Estate will run smoothly at 25mph in OD (4.56 axle) on the flat ,but you can forget about accelerating. It does mean that I often break the above rule and click into OD at a lower speed.
Save up and fit an OD, it's worth it.

[c1800]

The late 1800’s, E and ES, ( Late Amazons too, I think) used 4.30 gearing. Likely a compromise, with a little loss of pull on the low end, and somewhat better cruising.

[Derek UK]

Wrong way round. A 4.30 will lower the gearing from a 4.1. Just a few of the last Amazons got a 4.30 axle. A bit unpredictable. It's not a bad compromise between a 4.10 and a 4.56 when thinking about performance v a good cruise if you do add an OD. Whatever suits your driving style. A 4.30 will be hard to find.

[P1800Ray]

Volvo_133 (Chris) I’ve tried to PM you but your box is full so I can’t.

Is your shaft for the J or D overdrive? I’d like to discuss if you can make some space in your inbox.
Thanks

[P1800Ray]

I think my 1966 Estate is supposed to have a 4.55 diff, are there any markings on the diffs to definitively identify?

[Volvo_133]

Hi,
gearbox items are for the D type unit. i now have space for messages.
Chris

[c1800]

Originally Posted by Derek UK

Wrong way round. A 4.30 will lower the gearing from a 4.1. Just a few of the last Amazons got a 4.30 axle. A bit unpredictable. It's not a bad compromise between a 4.10 and a 4.56 when thinking about performance v a good cruise if you do add an OD. Whatever suits your driving style. A 4.30 will be hard to find.

Yes, thanks for clarifying, I was thinking about the 4.56.

[Bracpan]

Yes a 4.1 or a 3.9 will lower the revs at higher speeds but not sure how high your 1st gear will be and if you can pull away clean.. probably ok on the flat but hill starts might be a problem.. and towing would be a no no ...
I agree your answer is to fit an overdrive, or adapt a modern 5 speed box making sure the 5th is a higher gear then 1:1....

[classicswede]

Condisering a 131 with the 4.1 will pull away on the flat in 4th gear upping the gearing on the axle is not really going to hurt unless you do traffic light racing or super heavy towing.