Can easily top 80 in my 2000 110sw and 70 is reasonably comfortable. (watching fuel gauge less comfortable) I'm on 235 70's with a road tread. Despite this I'm with @lightning and happy sticking to 60/65.

ONE LIFE LIVE IT. D90 said:

the TD5 is capable of cruising at motorway speeds

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Yes, as you say it is capable of motorway speeds. I can achieve them easily and it feels every bit as safe on the road as you'd expect so that box ticked.
Its noisy, sound deadening helps but a shed doing 70 has to work hard and has to shove a lot of wind out the way. The defender is a vehicle designed for practicality as a work/utility vehicle. That it has now become a school run favourite, every day shopping vehicle or motorway cruiser is no fault of its own. Land rover realised this years ago. The solid rugged work/utility vehicle it had created was just that, a work vehicle firstly but still reasonably adept away from the work situation. Things were changing and, recognising this they brought out the range rover followed by the defender and freelander giving 3 choices from budget to luxury of 4wd car/utility vehicle all equipped to be comfortable and refined (!!!) at motorway speeds and fairly adept off road too.
What I'm saying is "horses for courses". It does what it says it does. Its first and foremost a workhorse plus it can do other stuff.

My 2000 TD5 90 is kicking out 212bhp/444Nm with a D2 transfer box and 265/75R16's, and although it will do 110mph if pushed it starts becoming unhappy after 85mph. It will sit at 70 nicely on the motorway. Another consideration is the TD5's are quote revvy sounding - with a Nanocom or rev counter you would be surprised to see 2500rpm when it sounds and feels like 4500rpm!

We are running a 110 TD5 with a light remap and a Disco Box. It will cruise at 70mph at 2500 RPM (interestingly the same final ratio as 6th in my work Navara). At these speeds, although the revs are low the engine is noisy any higher and you can tell the increased effort. I guess wind resistance and speed is not a linear relationship.

I would say the disco box has made it more comfortable on the motorway, but the landy still tends to naturally sit at 65.

This is one of them there "should have bought a freelander" freds

Brown said:

Yes, I've had a rattly jingly noise on mine which is more obvious at speed. Sounded a bit like a load of loose washers on a bolt being shaken vigorously. Mine quietened down considerably when I did the clutch and replaced the dual mass flywheel. The old one was very lose and rattly by comparison with the new one which was much tighter. What also disappeared were some alarming groaning and squeaking noises which used to occur on cold damp mornings. People usually point to the clutch release bearing and the bronze bush in the back of the crankshaft, but mine appeared to be as good as new. Another thing which was contributing was the bonnet rubbing on the wing. You can't really adjust it so I put grease on the point of contact and that's now a bit quieter.

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You can adjust the bonnet height at the front if it’s touching the wing.
The catch can be adjusted for height and then the rubber stops need adjustment as well.
At the rear there’s less scope for adjustment but you can loosen the bolt on the bulkhead at the rear of the wings and lower the wing top slightly.
Which may be enough to stop the bonnet rubbing on the wing tops.

MungoPark said:

I guess wind resistance and speed is not a linear relationship.

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Very good guess. It is a logarithmic relationship, wind resistance increases dramatically over about 55 mph, as anyone who has ever ridden a motorcycle knows.

lightning said:

You can adjust the bonnet height at the front if it’s touching the wing.
The catch can be adjusted for height and then the rubber stops need adjustment as well.
At the rear there’s less scope for adjustment but you can loosen the bolt on the bulkhead at the rear of the wings and lower the wing top slightly.
Which may be enough to stop the bonnet rubbing on the wing tops.

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Loads of clearance at the front. It's at the bulkhead end where it touches. There's still plenty of rubber in the hinges, and the wings won't go further down. If I really wanted a podium position at the waste-of-time Olympics I could try bending the hinges on the bulkhead up a bit, but I haven't quite got round to that yet!

MungoPark said:

I guess wind resistance and speed is not a linear relationship.

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Simply put, generally as speed doubles, drag quadruples. On planet Earth anyway. I wouldn't expect much drag in space.

wireman said:

When I first got my td5 90, it revved so much at anything over 50, I thought I was in 3rd gear! I fitted a discovery transfer box now it can sit at 65/70 all day.

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in addition to this and all the messages about sound proofing, the Defender has a slightly lower 5th gear ratio to the Disco. You can swap this over, this could help drop the Revs a little up at higher speeds on the motorway.