You'll get 13% increase, by the maths, using a disco transfer box, but that won't translate into a real world economy gain of 13% unless you drive at the same speeds you used to (not the speed shown on the speeo though - that will be out by 13%) - aerodynamics aren't exactly a defenders strong point.
In the 'real world' you'll probably only get a 5% economy gain from doing either - although it will cruise a lot more comfortably at motorway speeds.
Worth bearing in mind, with either of the above, if you increase the gearing without boosting the torque the car will accelerate slower than before. You probably won't notice around town, but you will on motorway slip roads and climbing long hills.
Cheers,