overdrive units

[clips]

any one with any experience of overdrives?
 how much difference do they make? are they worth the money? are GKN the ones to have or is there a solid, effective alternative, preferably cheeper, but alternative anyhow.
 and could i with next to no car diy knowledge fit one? or is pro fitting reccomended.
think the GKN was designed in conjunction with landrover, but never an approved part, and so attatches to the side of the gear box and runs through a worm drive or some driven attatchment, now im just confusing myself...... help, please, anyone :)

(still under warranty, so dont want to change whole gear box for different ratio gearing from say ashcroft)

[defenderdan]

I had a GKN overdrive fitted to my 300tdi 90 and it was brilliant. Don't expect to notice any improvment in fuel economy, but the noise reduction due to lower revs is great. It makes the vehicle more drivable especially when towing because you effectively have an intermediate gear. I let mine go when i sold the vehicle and i really miss it. I can't comment on ease of fitting because i let someone else do it.