David ,
when BCL fitted overdrives to 405 and 406 they used a 28% reduction overdrive and fitted the 4.2 ratio diff (approx 9% difference) , so the overall
ratoi difference was about 19%.
I've been told the current overdrive units used are about 18% reduction which equates to about the same .
With my second 400 I have been trialling a Ford type 9 5 speed gear box .The 5th gear is 18% reduction to 4th . I have only covered about 6,000 miles in this car but found at the 70mph max speed limit in Australia the engine is happier in 4th gear than 5th . Later this year I will be changing this box for a B/W CR5 with remote shift as they seem to be a better box than the Ford.
The B/W wont have the 5th gear but for my useage it is no loss.
Your 400 with a 4.2 ratio 405 diff will need either the original 3.9 diff fitted or an overdrive.
If you have a complete 85C engine , rebuild that and keep your original 85 as a spare.
Geoff
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