403 J Type Overdrive
The J type suited the 403 rear axle and at 100 kph reduced the engine revs by about 500 i.e from 2500 to 2000 approximately. It is only possible to engage it in top gear due to an electrical cut-out switch which was incorporated on the gearbox. On the other hand, the 405 overdrive had different gearing and required the 4.2 ratio diff to suit that gearing.
As far as I can see, the Laycock de Normanville J Type overdrive system was devised to reduce engine revs/increase speed. On the 403 a steep incline required the overdrive to be taken out of gear to increase engine revs rather than strain the engine in top.
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