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My GT has the stock drive-train - 2.0l 907, Getrag 235/5, 3.45:1 Magnum 2300 axle. I want a proper overdrive to reduce the revs on long distance journeys. The "standard" solution seems to be the W58 gearbox upgrade with the 3.73:1 Viva 1800 axle. I would, however, prefer to retain the 235/5, as I think its dogleg 1st and close ratios are part of the car's uniquely quirky character.
As far as I know, there is no Getrag box with a dogleg 1st and a taller 5th gear. I looked quite seriously at a standalone Layock o/d unit, but there is not enough room in the transmission tunnel.
In parallel I am going to upgrade the engine to Lotus Spec 10 (2.2l, HC, 180bhp). I am wondering whether the extra power and torque might allow me to get away with a taller diff ratio, dropping revs on the motorway without sacrificing acceleration at lower revs.
There does not seem to be a taller cwp set available for the Magnum 2300 axle off the shelf. A custom set would probably be prohibitively expensive. An alternative option would be the Ford Atlas axle from a V6 Capri, which has a 3.09:1 cwp set. This axle has been fitted to JH's in the past, with relatively minor modifications required.
Does anyone have any other ideas for achieving a taller diff ratio? Would 3.09:1 blunt lower down performance too much, even with a 2.2l HC conversion?
Any thoughts / advice would be greatly appreciated.