Very nice and well made overdrive systems for 90/110/Disco/Range. The unit drops the ratio by 26% and is shifted hydraulically/electrically even under load. It fits on the back of the L230T Transfer Box. Mounting time for Defenders 2 hours, for Discos 4 hours. The unit costs 995 GBP plus VAT. There was a cut-away on the stand at Billing and it looked quite serious. Looks like a good addition if you have fitted short diff gears.
Left: Cut-away view of box and T-case. On top new gear switch knob
In 1999 GKN started to manufacture an overdrive unit of their own. It features an 28.3% reduction in revs, fuel economy is said to be reduced by 20% (?). Other features are similar to Paynes system featured above as is the price of around 1000£.
Below: Chematics of the overdrive's working.