Mark V - X 420G 1948 - 1970

Gearing mk 10/420g

 
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Old 07-24-2018, 12:58 PM
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Default Gearing mk 10/420g

I have a 40,000 mileage 420G with the BW 3 speed auto box. I just want to get an idea of what sort of speeds should produce what revs as the car has a really high rev at motorway speeds.

As I pull away and accelerate, it feels that I get two gear changes, one around 10mph then another at 20-25mph then that is it. As a result, the car is doing approximately 4500 revs for 65mph - which is almost red lining and I'm sure is completely wrong.

The car has had some changes over the years (No major changes under my watch, except electronic ignition) and I don't know if the rear diff may have been changed? I'm not a great technical man, so any suggestions may have to be explained with kid gloves, but I chatted to a Bentley owner the other day who also had the 3 speed and he said he cruised at 70mph with 1800 revs, (I appreciate different engines/torque) but it seems a much more appropriate level!

Someone has suggested that it may not be moving up into third gear - but I do feel two changes - should there be three changes?

Any suggestions would be gratefully received!
 
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Old 07-24-2018, 04:20 PM
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Apart from somebody fitting completely the wrong diff, the only other thing could be the torque converter is slipping due to wear in the one-way clutch. Then there are the brake bands in the gearbox, these might need adjusting but that's a rather longer shot.
 
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Old 07-24-2018, 06:09 PM
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Its a three speed box so starting in first you are only going to feel the 1-2 and 2-3 changes, having said that 4,500 for 65mph is not right, my S11 with 4.2 engine is doing <> 2,800 a 60mph, this will have the same 1-1 top gear ratio and approximately the same diff ratio as the MK10/420
 
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Old 07-25-2018, 02:18 PM
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Thanks for the replies - first stop... Looking at the rear diff and finding out what it actually is...
 
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Old 07-26-2018, 09:32 PM
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Default BW8 shift info

From the Mark X manual:

In D1: Upshift from 1st to 2nd at 6-7, 2nd to 3rd at 10-12. Starting in D2, shift into 3rd at 10-12. These are both for minimum acceleration.
The BW8 has a 3.54:1 rear axle ratio. The manual shows 2780 rpm at 60 and 3241 at 70mph. My Mark X tach shows 3200 at 68 mph.

Phil
 
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Old 04-20-2019, 10:26 AM
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Stupid related posts. Mods, please make this “feature” optional!
 
 
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