I've driven Defender TDCI 110s on the motorway at 80 without any issue whatsoever... the TDCI model seems to also be bolted together a bit tighter than the TD5 (might just be because they were newer vehicles, but much less slack in the steering, etc) and if you're doing lots of miles the slightly more modern dashboard, and ability to have air-con without losing leg room in the front is quite useful. The seats are also more comfortbale.