Will try to do this objectively, DH and I want to know who's right! I will try to not reveal whether I am driver or passenger 
Only one of us drivers, the other has a driving license but doesn't enjoy driving, hasn't driven since age 25ish (about 17 years). This isn't a marital bone of contention as the driver loves driving and isn't wild on being a passenger. We live in London so don't use the car much for short journeys at home, mainly for longer trips away at weekend - seeing family, days out etc.
The driver is generally a good and safe driver. Has held a license for 20 years, only once got a speeding penalty aged 20 (3 points long since elapsed for motorway speeding). Nevertheless, they regularly drive 5% over the speed limit on motorways for short periods, sometimes 10%, and this irritates the passenger.
Who IBU?
The driver maintains that it's very difficult to hold an exact speed and that they aim for 70 on motorways but this fluctuates around 65-75 quite naturally due to variations in gradient, road surface etc and that driving is basically a series of constant minor adjustments. So that it's not U to go over 70 for a short period as long as you're keeping a watchful eye on it and bringing it back down to 70. They maintain that by the time the passenger notices they're speeding, the driver has usually already noticed and is in the process of adjusting downwards. But due to having had a couple of motorway driving lessons when 17 immediately after passing test, driver is convinced that it is Very Bad Practice to use the brakes to adjust speed on the motorway. By preference, speed and spacing between cars should be adjusted only by varying acceleration (obviously this doesn't apply if you need to brake to avoid hitting someone). They were taught it is both dangerous and causes unnecessary traffic jams when cars brake so they avoid it where possible. The result of that is it sometimes it takes a few seconds - occasionally maybe 10 or 15 seconds depending on the gradient - to slow back down to 70 without braking. Using the same logic, the driver tends not to brake for speed cameras, although they will adjust downwards without braking if they can, reasoning that if they get a ticket then they deserve one and that using 'unsafe practices' (braking for no reason) on a motorway to avoid a ticket is unacceptable. Both agree that driver doesn't persistently speed (i.e if they notice they're speeding they always take action to bring it down, don't just cruise along at 75).
Passenger maintains that the speed limit is the speed limit, end of, and that if the driver ever finds themselves above it they should slow down immediately (braking if necessary). And if the driver is so insistent on a process of constant adjustment and apparently unavoidable speed variation then they should aim for 65 rather than 70 so as to avoid going over 70, ever.
Driver thinks aiming for 65 is quite wrong, that the highway code says you should drive up to the speed limit unless road or weather conditions make that unsafe. In fact, driver failed their first driving test for persistently driving 20-25mph in a 30mph zone.
Passenger is also irritated by their partner's cavalier attitude to getting tickets (and thinks that the fact the driver hasn't got a ticket in 18 years is luck rather than judgement).
In case relevant, both parties agree driver is very careful in residential areas with speed limits, this is just a motorway issue.
So - Mumsnet jury? Should the driver slow the fuck down, aim for 65, brake for speed cameras and generally stop being an arrogant twat to justify their casual minor speeding?
Or is it bit rich for the passenger to be back seat driving given they haven't actually been behind the wheel of a car for nearly 20 years?