The gearboxes on diesels will often be a bit taller geared than the equivalent petrols in order to compensate for the woefully inadequate rev range that the
burners are limited to.
I thought the best indication would be to compare similarly powerful cars of similar size and weights, one a Renault with the 6 speed box and another from a different manufacturer with the 5 speed. So I went on the WhatTowcar website and compared my Lag II 1.9 dCi 130 with a Mondeo 2.0 TDCi, also rated at 130-ish bhp.
They still don't list the 1.9 130 Renault engine, so I chose the 120 and tweaked the power and torque figures using the 'chiptuning' options. Not that it makes any difference to gear ratios, but I wanted to compare similar cars.
Figures (Gear/Renault MPH@1000rpm/Ford MPH@1000rpm)
Identical to within 1mph for gears 1 to 5, for a similar size and weight of car with a similarly powered engine. Judging by that, the 6th gear (on the Lag at least) is very much an extra overdrive gear on top of the standard 5th.
This is also reflected in the WhatTowcar's rating on towing ability in top gear: The Lag rates poorly at one star in 6th, whereas the Mondeo manages four stars because it's tested in the lower ratio 5th.
As I already mentioned, 6th gear is a bit of a mixed blessing in this respect. If you were cruising on the motorway in 5th on the Mondeo and floored it, you'd get a healthy pick-up of speed. Try the same in the Lag and you'd be cruising in 6th, and it wouldn't be so willing to accelerate.