Is it actually wrong to double de-clutch when you change down? If so, why?
If you have an auto box … yes 
It upsets the syncro mesh.
and a rev up as you change down?
My assessor said so when I did my driving assessment.
He said it’s wasting diesel by revving up when doing a changedown.
I know it’s a hard habit to break when you were bought up on “Crash” 'boxes, but after driving 'em all for over 40 years, and being a fitter in between, I think that dipping the clutch, sliding down a gear, then letting the clutch out slowly again, costs more in synchro cone and clutch wear than it will ever cost in diesel.
But that’s just my opinion.
On my assessment it was easier to say,“OK, sorry”, than have a debate.
nodding donkey:
It upsets the syncro mesh.
Erm, erm, hmmmm?
On a synchro box it will always be good practice to match the revs to the gear selected before engaging the clutch, as above why put unecessary strain on the clutch/synchros, a constant mesh box required this anyway.
I wouldn’t double declutch on a synchro unless the cones on a particular gear were worn, when the gear can baulk badly or you get a grating of gears.
No hard and fast rules IMO, a driver should be able to feel their way round any manual box and adjust their methods accordingly.
“My assessor said so when I did my driving assessment.
He said it’s wasting diesel by revving up when doing a changedown.”
Bestbooties how did you control yourself, i’d have ended up crying tears of laughter at that howler.
Here in my left hand, i have a thimbleful of fuel ‘wasted’ by changing gear properly.
Here in my right hand are the umpteen thousands of pounds bills for yet another clutch, and then two clutches later an early gearbox rebuild.
I’ll take the left hand bill please.
There might be a strong clue as to why auto boxes are now ‘in’, no wonder manual box clutches last 5 minutes these days if that’s the typical standard of on the job training out there.
bestbooties:
and a rev up as you change down?
My assessor said so when I did my driving assessment.
He said it’s wasting diesel by revving up when doing a changedown.
I know it’s a hard habit to break when you were bought up on “Crash” 'boxes, but after driving 'em all for over 40 years, and being a fitter in between, I think that dipping the clutch, sliding down a gear, then letting the clutch out slowly again, costs more in synchro cone and clutch wear than it will ever cost in diesel.
But that’s just my opinion.
On my assessment it was easier to say,“OK, sorry”, than have a debate.
A decent double de clutched downshift contains two seperate,albeit effectively simultaneous,rev matching operations.The first one matches the input and output side gear train speeds in neutral and the second matches the road speed with the engine speed when the clutch is re engaged.The first of those,when used in a synchro box,can only help to take the load off of the synchros also thereby making the shift lighter and faster.While any assessor who says that the second doesn’t/shouldn’t apply,in the case of a synchro box,just the same as a non synchro,is talking bollox that is probably all about pandering to the idea of attracting the worst type of car drivers into the industry. 
I was in the car with someone last week, the type who for some reason goes down the gearbox one by one when slowing. Every single time the clutch was let out with no sympathy or attempt to smooth the release, with the result that it was a rough lurch on every downshift. It wasn’t even a company motor, it was her own. How on earth it doesn’t strike them that it may be causing unnecessary wear I have no idea, but there we are.
Who is still driving a manual■■? C’mon lads auto is just as easy once you get used to it
Trucker8oy:
Who is still driving a manual■■? C’mon lads auto is just as easy once you get used to it
And when you’re stuck in 3 miles of 4 lanes of nothing moving anywhere fast as the entire motorway is funnelled off the next exit and down backroads you’ll be damned glad you have an auto box. I miss having a manual gearbox so much on my truck I’m contemplating getting an automatic for my next car.
I often drive an old very high mileage motor that makes a really embarrassing noise if I dont double de-clutch it into 3 rd. gear.
alamcculloch:
I often drive an old very high mileage motor that makes a really embarrassing noise if I dont double de-clutch it into 3 rd. gear.
My missus does that as well! 
Conor:
Trucker8oy:
Who is still driving a manual■■? C’mon lads auto is just as easy once you get used to itAnd when you’re stuck in 3 miles of 4 lanes of nothing moving anywhere fast as the entire motorway is funnelled off the next exit and down backroads you’ll be damned glad you have an auto box. I miss having a manual gearbox so much on my truck I’m contemplating getting an automatic for my next car.
Ironically for the topic the best autos are constant mesh automated manual which needs to be synchronised by way of auto rev matching. 
Although just as ironically an automated manual doesn’t have a clue as to clutch biting points and slip amounts while shunting in the yard or crawling along in those traffic jams.
Which just leaves the compromised torque converter auto which is great for crawling along but drinks fuel and like the automated manual idea is an inevitable over complicated money pit as it get older.Often to the point where it is uneconomic to repair as opposed to break for parts.At which point if you do decide to buy that auto car be prepared to have to give it away when the time comes to fix it.Or to flog it.Either to someone who intends to break it, or convert it to manual.Bearing in mind that the latter is often now just as uneconomic as trying to fix it because of all the bs over complicated engine and transmission management interfacing.
All that just to save a bit of use of leg/foot and arm/hand.
Olog Hai:
I was in the car with someone last week, the type who for some reason goes down the gearbox one by one when slowing. Every single time the clutch was let out with no sympathy or attempt to smooth the release, with the result that it was a rough lurch on every downshift. It wasn’t even a company motor, it was her own. How on earth it doesn’t strike them that it may be causing unnecessary wear I have no idea, but there we are.
Easily answered 
If you are an old git like me, just keep on double de-clutching! It gives you guaranteed smooth, clean changes; it makes you plan your drive into all hazards; it almost certainly doesn’t do any harm to synchro-cones or any other components and it doesn’t waste diesel. If you were taught to drive properly then just enjoy driving properly. Robert
I seem to remember reading, a LONG while ago now, that double declutching a Scania gearbox led to overloading something internally that could cause damage? Think it must have been in Truck and Driver mag when that was available as I bought it occasionally, that would be in the 1990’s.
Pete.
Euro:
Is it actually wrong to double de-clutch when you change down? If so, why?
NO
robert1952:
If you are an old git like me, just keep on double de-clutching! It gives you guaranteed smooth, clean changes; it makes you plan your drive into all hazards; it almost certainly doesn’t do any harm to synchro-cones or any other components and it doesn’t waste diesel. If you were taught to drive properly then just enjoy driving properly. Robert
This. Because I’m too lazy to type my own spin.
windrush:
I seem to remember reading, a LONG while ago now, that double declutching a Scania gearbox led to overloading something internally that could cause damage? Think it must have been in Truck and Driver mag when that was available as I bought it occasionally, that would be in the 1990’s.Pete.
There was a similar discussion long ago which is still in the search system.The idea of arguably more load on synchros was described by an Eaton rep and seemed to be based on the erroneous idea that the shift into neutral in some way involves the engagement of the next gear which it plainly doesn’t.It’s a seperate move which is carried out anyway on a single clutched shift.The only difference being that the mismatch between input and output side is removed my rev matching in neutral with the clutch engaged.Which can do nothing but good in terms of lightening the load on the synchros.
On many of these modern AMT’s, (Automated manual transmissions), there are no synchro cones as a means of saving weight? but the computer controlling the 'box gets the revs JUST right to make every change smooth and silent, including the burst of revs on a changedown.