My 2.5 n/a 90 struggles to get over 45 at times, its on bigger 110 tyres and a bit over geared, have run it at 65 with a Disco on a trailer behind me on the motorway but had a following wind.
Motorway its ok at 50 / 55 but hills kill it and I imagine the petrol would be the same with defender diffs.
I think it will be ok with series diffs because I spend more time in 4th allowing the engine to run on a light throttle without too much load, it will be more economical despite being higher revving...., not enough power for high gearing.