To overdrive or not...

I have a 1968 Series IIA 2.25 diesel which belonged to my dad. It is completely original. Dad bought it in 1975 from a neighbour and it worked on the farm up to about 1990 and then just driven occasionally as the rear springs went up through the chassis. I have it completely stripped and am almost ready to rebuild it. I am in no panic and intend to restore it over a year or two. A new galvanised chassis and bulkhead are ready to go.

It is my intention to keep it as original as possible yet rebuild it like new. I don’t intend to off-road very much as it is going to cost a lot to get it going and I have a few long Sunday runs planned for it. Would you recommend adding an overdrive for a little more speed and if so, what unit would you recommend?

John Mcauley

ED EVANS REPLIES: The original fitment was a Fairey overdrive and, although these are now relatively rare, it’s worth looking for a secondhand unit. An overdrive won’t really give you a significant speed increase but it will, of course, lower the engine revs for a given speed, and in a diesel-engined Series IIA that will certainly quieten things down and give more relaxed cruising, and this is the main benefit. It’s very easy to fit and makes a nice period piece.

Alternatively, you can buy a new Roamerdrive, though it will cost over £2000. It’s part number DA9015; a search engine will find it. Both types are manually engaged using a lever in the cab, and both use the proven epicyclic gear system. The gearing increase is significant at 28 per cent, so it’s important to avoid labouring the engine at low revs. It’s been suggested they reduce fuel consumption but, at 60mph for example, the old diesel is still doing the same amount of work regardless of revs, and is not necessarily any more efficient at lower revs for a given speed.