If you live in Wales

If it was safety as they claim, then banning SUVs would have been a better idea, 

SUVs kill more pedestrians in accidents, yet they are sold as safer, which is also true, drivers and passengers are more likely to survive a crash.
and being honest the SUV owners I know round here all use them for the school run or to commute to work.

IMHO the 20mph idea has caused more risky driving I get far more cars now jumping out of side roads in front of me. The idiot group are still doing 40 and are overtaking more. the one minute on a journey is a fallacy unless that journey in town is less than about 1.5 miles.

the proposal that buses and taxis will still be able to do 30 is a farce, especially when comment was made yesterday that no exemption will be made for 999 vehicles on a call out

I'm thinking real World and a particular journey I make that is definitely 5 minutes or more slower despite most of it being National speed limit

Forgive me for observing that - in the absence of any other evidence - that is at risk of confusing correlation with causation. There is a stack of evidence from properly formulated studies and modelling that where there is heavy traffic load, reducing the maximum speed of traffic increases the average speed and the flow of traffic (aka reduces congestion). This is the reason for the use of variable speed limits on motorways in sections/areas particularly prone to high traffic loads. Or indeed fixed lower limits in some areas (e.g. M3 junction 2 where the M25 joins, and where a permanent 50mph speed limit was put in place at around the merge to reduce congestion/improve flow/improve safety). 

The results from both modelling and the real world in these situations is unarguable: reduced max speed leads to less erratic speed changes leads to less sudden hard braking and less bunching etc etc. which ultimately results in smoother traffic flows, higher average speeds, and higher flow rates. 

I would dispute the bunching up and braking is better. On the M1 you get changes from no limit to 60 to 40 back to 60 back to 40, trying to drive sensibly is impossible, you can't brake to 40 because of everyone around you is doing 60 then it goes back up to 60 and everyone accelerates and then back down to 40. Who is changing these limits, they are absolutely stupid.

sorry, but that is not so much 'blindlingly obvious' as 'blindingly obviously flawed'. What you appear to be trying to say, and extrapolating from, is that "going slower" is de facto less fuel efficient than "going faster". Whatever the relative efficiency of 20 mph vs 30 mph for any specific vehicle, it doesn't follow based on the merits of going at 1 mph vs 60 mph. The comparison/extrapolation is simply daft. If it was, then driving my vehicle at 150mph down the motorway would be more fuel efficient for a given distance than going at 40mph. And that ain't the case. It fails the sense check.

There are probably 3 primary factors affecting fuel economy for a given vehicle (at the same weight):

1. Rolling resistance
2. Drag
3. internal/powertrain losses/efficiencies

The first is going to be linear with speed, give or take. I think. Although at the margins that won't be the case (e.g. differences in tyre temperatures at different speeds). 

Drag on the other hand is not linear: it is dependant on the square of velocity. A body travelling through the air at 40 mph is subject to 4 times the drag of one doing 20 mph. And therefore the system has to do 4 times the work to overcome the increase in drag for the doubling in speed. Or one at 30 mph is subject to x2.25 the drag of one travelling at 20mph*. 

The 3rd has a lot to do with the design of the engine and associated components: i.e. what revs/power output/torque/speed envelope has the design been optimised for.

Self evidently, what speed envelope you are talking about affects which is the more dominant factor. At low speeds, 1 and 3 will be. At higher speeds the second will start to dominate i.e. so that higher speeds will lead to a decrease in overall efficiency. If that wasn't the case, rather than imposing speed limits of 50mph during the 70's oil crisis, everyone would have been encouraged to max out down the motorway and treat it like the autobahn. 


*[In the same way the kinetic energy of the moving body is also relative to the square of velocity, and hence the whole core argument around 20 mph vs 30 mph from a pedestrian safety POV: i.e. a vehicle hitting someone at 30mph has >2x the kinetic energy than if it was doing 20mph].

Pah, what do Imperial College know about science anyway?
www.bristol20mph.co.uk/wp-content/uploads/2016/06/speed-restriction-air-quality-report-2013-for-web.pdf

OTOH, there's some feels... They're far more important than science.

The whole 56mph "most fuel efficient speed" myth is simply that there used to be three fuel consumption figures quoted in UK car ads. Urban, 56mph, 70mph. Three guesses which was best?

But why an odd non-round number like 56mph, if it's not some magical sweet spot? Simples. It's 90kph. A typical pan-European extra-urban non-motorway cruise. No more scientific than that.

And, no, cars aren't all tuned to rig that figure. It was simply not possible to do that back in the days of carburettors. Those fixed-speed consumption figures went out the window decades ago, in favour of actual simulated driving cycles. Sometime around, ooh, the time that electronic fuel injection (which might perhaps have allowed something along those lines) and cats (which don't like the selective leaning-off that it'd need) became universal.

It gets worse.  Perhaps counter-intuitively, the power required of the engine actually goes up with the cube of velocity rather than the square.  To double your speed requires 8 times the power. 

Drag accounts for the second power, the quadrupling of work done to overcome air resistance, as you've said.  But you then have to add the cost associated with moving the vehicle from A to B in half the time.  4x the work done (to overcome drag) requires 4x the power, but 4x the work done in half the time requires 8x the power.