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Premium diesel? Additives? Neither?

Lorries in the UK can't get to 70mph. They have speed limiters.

So that’s a good idea then, as one creeps past another ?
I didn't comment on whether it's a good idea, I'm just pointing out a fact. The speed limit for HGVs on a Mway is 60, they have limiters to 56. Is this a good idea? Well, if the alternative is lorries routinely breaking the speed limit, speed limiters are probably the least bad alterative. If you think it's a bad idea, what would you rather and why?

Bear in mind that if an HGV driver/mechanic did disable the limiter and the police pulled him over, the first thing they would do is look at the tacho. If that shows routine speeding or any signs of tampering, the next step is an engineer's inspection to see if the limiter is still in place. No? The vehicle operator and driver can look forward to their day in court then. Illegal vehicle mods I'm sure are very well received by the local beak.

There's a way to avoid problems with HGVs on Mways and dual carriageways without breaking any speed limits. Set your speed around 65, engage cruise control if you have it, and sit back. You'll be able to overtake all the HGVs you want without them getting in your way or you lingering too long in Lane 2 or Lane 3 , anybody wanting to go faster can overtake you whenever you like, and you'll get great fuel economy. I did this this morning on the M1 past Wakefield, the journey passed without incident.

Regards, an Audi driver.
 
The environmental issues with raising the U.K. speed limits will alway win. Fuel consumption at 80mph is noticeably higher than at 70.
And it’s not just emissions from the exhaust that concern us. Tyre and brake debris come into the equation now.
I know from my petrol car (ok, a powerful mid-sized SUV so aero is a bigger factor), a 60mph cruise results in 50mpg, 70mph gives 38mpg, 80mpg results in 32mpg.
 
Aero… it didn’t occur to me consciously but I had a toss up between two Volvos, a V60 & XC60. The latter was impressive visually but it’s a brick basically. The latter, which I chose, does look slippery. It’s odd being lower to the ground again but I only really notice it when turning left on a curbed road. Is the lower profile worth a couple of MPG all other things being equal?
 
Fizzics:
  • Aero drag = Cd x A (frontal area, sq.m)
  • Power to overcome drag varies as v^3.
... there's also a small linear term that describes rolling-resistance, but that is in the weeds / c falling below 20% above c.50mph.
And fuel consumption is dominated by the power output required.

summary - smaller CSA profile makes a massive difference, at speed. Esp if you were comparing two cars from same maker, with likely same engine.
 
Looking at the Honest John website, owner feedback for fuel consumption appears to show the XC uses about 10% more fuel than a V with same/similar engine. Not a huge difference, until you can’t afford your gas bill in February and your commute is 2,000 miles per month.
 
I may be wrong but my aged brain is suggesting that the 70 mph limit was bought in during a fuel crisis as an economy measure.

Dave
 
During the fuel crisis it was dropped to, IIRC, 50mph.

Worth recalling that at the time 70mph was deemed acceptably safe on motorways, the majority of cars used non-assisted drum brakes and crossply tyres, and were pretty badly maintained. 70mph was close to the top speed of most of them, too. Braking distances are now considerably shorter than in those days, and ABS means cars can manoeuvre under braking to avoid hazards. So 70mph is now much safer speed than it used to be, even allowing for the increased traffic levels.
 
I’ve noticed that as fuel has become more expensive the number of people wizzing past me has vastly decreased. 60 mph does not seem unusual any more. Most cars round the M60 seem to be going just a little bit faster than the lorries. The M602 has a new speed limit of 60 to reduce emissions. I’m happy to drop the speed a bit, it massively extends the range of my car, and doesn’t seem to take much longer.
 
I’ve noticed that as fuel has become more expensive the number of people wizzing past me has vastly decreased. 60 mph does not seem unusual any more. Most cars round the M60 seem to be going just a little bit faster than the lorries. The M602 has a new speed limit of 60 to reduce emissions. I’m happy to drop the speed a bit, it massively extends the range of my car, and doesn’t seem to take much longer.
Yes, I've noticed this, too. And more people seem to be using cruise control. As with lorries taking an age to pass in the middle lane, we now get cars toing 70.0mph in the outside lane, passing traffic doing 69.9mph in the middle lane.
 
But the weak point remains, after all these years.
True enough, and I don't suggest that thinking distances have dropped - indeed they may have got worse in some cases due to drivers being in a lower state of arousal (quieter cars, cruise control, lane keeping, etc). But both active and passive safety have improved massively since the 70mph limit was introduced. I won't use that as an argument to raise the limit, but I will say that those who suggest it as an inviolable limit are perhaps taking things a bit too far if they argue safety as their basis for that.
 
True enough, and I don't suggest that thinking distances have dropped - indeed they may have got worse in some cases due to drivers being in a lower state of arousal (quieter cars, cruise control, lane keeping, etc). But both active and passive safety have improved massively since the 70mph limit was introduced. I won't use that as an argument to raise the limit, but I will say that those who suggest it as an inviolable limit are perhaps taking things a bit too far if they argue safety as their basis for that.
I agree.

And the tendency is for the police (not speed scameras) to allow a little leeway based on common sense and whether speed wandered a little high for only a few seconds.

I’ve been a passenger in cars where the drivers refused to believe that on a dual carriageway the limit for their car was 70mph, not 60. And I can state that 60mph felt less safe.
 
I agree.

And the tendency is for the police (not speed scameras) to allow a little leeway based on common sense and whether speed wandered a little high for only a few seconds.

I’ve been a passenger in cars where the drivers refused to believe that on a dual carriageway the limit for their car was 70mph, not 60. And I can state that 60mph felt less safe.
Yes, that 60mph thing is a common misunderstanding, too I think. The NPCC (nee ACPO) guidelines for speeding offences are that the threshold for prosecution will normally be limit + 10% + 2mph, which means that leeway is given for anything up to 79mph in a 70 limit, unless conditions suggest otherwise. Normally, also, speeding at or within another mph or two will give rise to an invitation to a speed awareness course, rather than points so the 'practical' limit for motorways is 80mph or thereabouts, road conditions permitting. This feels to me like a typically British fudge: politicians were reluctant to raise the limit closer to continental limits (130kph being a touch over 80mph) so instead the 'enforcement' limit was set so as to create a 'de facto' higher limit for practical purposes. The absolutists will have none of it, obviously, but that's my take on things.

I also think that the +10% +2mph thing gives more leeway at urban speeds than is ideal- 35mph is 17% faster than the limit, whereas 79mph is only about 13% higher. I think 35 is usually too fast in an urban context, whereas 79 feels fine in many motorway situations.
 
All the towns in the Borders have had their limit dropped to 20mph. I found it a little annoying at first, but now I’m used to it I’m definitely in favour. Should be introduced everywhere.
 


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