Jon - Sorry if quoting your previous posts doesn't read correctly, I did a search and your posts were the ones that came up, as your quite knowledgable/experienced on the subject. (Feel free to edit the post so that it doesn't cause any confusion).
The point I wanted to make Steve aware of, was that the diff ratios are different between a manual and an auto, something that I learned on this forum. So that if Steve was to install a manual gearbox in place of his faulty auto gearbox, he wouldn't be taken by surprise when he found that his 80 was no longer geared as Mr T had intended.
Chris - A manual running through 4.1 diffs and running standard size tyres, would cause the engine to revolve faster for a given road speed. So it would cause the engine to rev faster at motorway speeds than it did originally.