Gearbox - 6th Overrated

JJJJNR

Had a discussion with a mechanic, who says pottering about at 60km/h - 100km/h in 6th all the time is bad for the gearbox and its best to be in 4th to 95km/h 5th to about 120+ and not to use 6th as it causes a lot of wear on the gearbox at low speeds (car is a SEAT Altea 2.0TDI 140BHP) .

I've been experimenting for the past couple of weeks and have noticed there's a huge difference in economy if I do stick to 4th up to 95km/h and don't change up early which I've been doing I'm wondering though would driving at that speed in that gear have a negative effect on the turbo, as its sitting at 2700 RPM where usually in 6th it would be at around 1200rpm.

Economy wise computer says on my usual 50km commute to work its hitting 3.8 per/100 usually in 6th it would be about 4.7 per/100.. while doing an avg speed of 100km/h (or 90 - 120km/h depending on conditions etc)

So question is would it have any long term effect on the car, if I do change my driving style and what damage or wear would driving at low speeds in 6th cause

smash

I think it will hit your fuel bill more than anything else. The gear is there so the engine isn't as stressed.

L-M

My cousin drives a fleet Insignia and finds that at 120 kmph it labours in 6th, so as a result he never actually uses it.

pred racer

At 100km/hr my car (alfa 1.9jtdm) is more economic in 5th.
Theres not a whole pile in it though, so I tend to stick it in 6th.
The guys on alfaowners seem to be of the opinion that driving the 159 in higher gears at low revs leads to increased clutch wear. I dont really know though.

Mycroft H

Mechanic is talking part rubbish, it really depends on the car and it's gear ratios. I've driven a laguna 6spd diesel and it happily potters on the motorway at 110km/h at around 1600rpm.

b318isp

It depends, some cars will run fine in 6th at 100-120 on the flat, but may struggle a little on slopes due to power/weight.

There is also an arguement not to heavily load dual mass flywheels at low revs on diesels as the torque may cause damage over time. Having being there myself, I avoid driving under 1500rpm on our 1.8TDCi and try to ensure at least 1900rpm before putting my foot down. This often means dropping from 6th to 5th for a climb.

keithclancy

My rule of thumb is if the car is struggling I need to change down a gear.

Depends on the Car Really.

Matt Simis

The wear he mentions is suggested as one of the causes of early DMF failure in Diesels, the juddering from an engine labouring (even slightly) is putting strain on the box that it wasnt designed for.

But the way its phrased as a hard and fast rule for all cars is incorrect. Bigger engines could pull from a rolling stop in 6th, let alone drive 120kph.

I wouldn't cruise at 2700rpm in a diesel if there was a gear to change up to.

166man

I'm driving a 159 diesel at the moment and I always cruise in 6th at 100-120km/h. It's far less stressful than 5th which feels more like a fourth gear. To get to 120km/h I use 3rd,4th,5th and then settle on the motorway is 6th. According to the on-board computer it's much more economical.

Anan1

Quite apart from any issues of wear, am I the only one who finds 6 speed manuals (and the narrow power bands that usually necessitate them) a PITA? It's like rowing the car along.

Yakuza

Here's a snippet from post I put up last year in relation to motorway speeds in Spain (read the thread for context) but here's the economy figures for a 318d running at 110km/h (by GPS, not the speedo):

Yakuza wrote: »

The car was a 318d (2010 I presume, hard to tell with Spanish plates, 70k km on the clock so well run-in ). Two adults, 3 kids and a full boot.

In 5th - the car was doing 2300 rpm and the instantaneous fuel consumption was hovering around 7 l/100km.

In 6th - 1900 rpm and roughly 5.9 l/100km (needle was just to the left of 6)

For that car at least, 6th gear meant lower revs and lower fuel consumption.

Mycroft H

Anan1 wrote: »

Quite apart from any issues of wear, am I the only one who finds 6 speed manuals (and the narrow power bands that usually necessitate them) a PITA? It's like rowing the car along.

I love them. Decreased fuel consumption for the win.

Matt Simis

Anan1 wrote: »

Quite apart from any issues of wear, am I the only one who finds 6 speed manuals (and the narrow power bands that usually necessitate them) a PITA? It's like rowing the car along.

I told you before, yes you are!

The 6th gear is just the overdrive, its a negative torque multiplier. My 6sp 540i has the same gearing as the 5sp M3 box only the top gear in the 5sp is 1.0 and the top gear (the extra 6th gear) in the 6sp is 0.83. Thats it.

It in no way detracts from the car or engine or drive, the sole purpose is to lower RPM (at cruise, exactly the same outside 6th/motorway) in abundantly powerful cars. 6sp in a Punto Sport on the otherhand I assume is pointless, but I would be surprised if they reduced torque multiplier across the board to fit it in, again its most likely the 6th gear sits on top of the 5th, otherwise the same.

pajo1981

Anan1 wrote: »

Quite apart from any issues of wear, am I the only one who finds 6 speed manuals (and the narrow power bands that usually necessitate them) a PITA? It's like rowing the car along.

6th is usually an overdrive gear- so no different than a 5 speed, except with the extra eco-gear.

Anan1

Matt Simis wrote: »

I told you before, yes you are!

The 6th gear is just the overdrive, its a negative torque multiplier. My 6sp 540i has the same gearing as the 5sp M3 box only the top gear in the 5sp is 1.0 and the top gear (the extra 6th gear) in the 6sp is 0.83. Thats it.

It in no way detracts from the car or engine or drive, the sole purpose is to lower RPM (at cruise, exactly the same outside 6th/motorway) in abundantly powerful cars. 6sp in a Punto Sport on the otherhand I assume is pointless, but I would be surprised if they reduced torque multiplier across the board to fit it in, again its most likely the 6th gear sits on top of the 5th, otherwise the same.

I get that, but how far do you go with it? Would 7 forward speeds bother you? How about 8? Or 9? Because 5 and a proper engine has always seemed enough to me.

pajo1981 wrote: »

6th is usually an overdrive gear- so no different than a 5 speed, except with the extra eco-gear.

The difference is one more gearchange!

pred racer

166man wrote: »

I'm driving a 159 diesel at the moment and I always cruise in 6th at 100-120km/h. It's far less stressful than 5th which feels more like a fourth gear. To get to 120km/h I use 3rd,4th,5th and then settle on the motorway is 6th. According to the on-board computer it's much more economical.

At 100, 5th is a tad more economical, its revving a bit more, but not boosting as much to get up small hills, but your right, it doesnt sound 'right'

Normally i find myself at 120+ in 4th, have a little smile to myself;)
Then its 5th, 6th on the way back to 100-ish.

Matt Simis

Anan1 wrote: »

I get that, but how far do you go with it? Would 7 forward speeds bother you? How about 8? Or 9? Because 5 and a proper engine has always seemed enough to me. :DThe difference is one more gearchange!

Well, when you have something you know would drive fine at 1000rpm and you are at 2800rpm in 6th on a long empty motorway, yes I think I could use a 7th. An alternative solution would be a much more aggressively torque reduced 6th.

The difference with adding a 7th vs a 6th however is physically a gearbox already has 6 notches and reverse is easily positioned as a special case.
Not quite as neat:

LIGHTNING wrote: »

I would have assumed in a car like the Punto it allows you to have shorter ratio`s in the first 5 gears for acceleration and then have a longer 6th gear for cruising?

Perhaps and again a good use of more gears in a torque restricted car.

ThisRegard

Limerick man wrote: »

My cousin drives a fleet Insignia and finds that at 120 kmph it labours in 6th, so as a result he never actually uses it.

What engine ? Our 2 litre diesel doesn't labour in 6th at that speed, it gets around quite fine.

Anan1

Matt Simis wrote: »

Well, when you have something you know would drive fine at 1000rpm and you are at 2800rpm in 6th on a long empty motorway, yes I think I could use a 7th. An alternative solution would be a much more aggressively torque reduced 6th.

The difference with adding a 7th vs a 6th however is physically a gearbox already has 6 notches and reverse is easily positioned as a special case.
Not quite as neat:

I rarely top 180km/h these days, so I don't think that 7th would be of much use to me. But why restrict yourself to an old-style gearlever? With an automated clutch you could have 8 or 10 forward gears, even more if you wanted. What's the ideal number?

pajo1981

Anan1 wrote: »

TH difference is one more gear change!

It's not, because to get to the equivalent of a 5-speed's top gear still only requires 5 gear changes.

If you don't like the 6th gear just pretend it's not there and... voila, instant 5-speeder.

keithclancy

Anan1 wrote: »

I get that, but how far do you go with it? Would 7 forward speeds bother you? How about 8? Or 9? Because 5 and a proper engine has always seemed enough to me. :DThe difference is one more gearchange!

You taking the píss out of my Legs ... my Bicycle has 18 gears ... :pac::pac:

pred racer

Anan1 wrote: »

I rarely top 180km/h these days, so I don't think that 7th would be of much use to me. But why restrict yourself to an old-style gearlever? With an automated clutch you could have 8 or 10 forward gears, even more if you wanted. What's the ideal number?

As many as the little dirt bike in Terminator 2, we counted and Iirc there were 18 up shifts during the chase scene:D

ShadowHearth

I got only 5

Anan1

keithclancy wrote: »

You taking the píss out of my Legs ... my Bicycle has 18 gears ... :pac::pac:

I know nothing about the width of your Power Band.

Matt Simis

Anan1 wrote: »

I rarely top 180km/h these days, so I don't think that 7th would be of much use to me. But why restrict yourself to an old-style gearlever? With an automated clutch you could have 8 or 10 forward gears, even more if you wanted. What's the ideal number?

I think you are making out this is more PR and marketing than a technical requirement, I see it the opposite:

The ideal number is the gear/torque reduction that gives the lowest drivable RPM at a target expected cruise speed.

Looking at a stock 850CSi:

Lets assume a top crusing speed is 140kph across Europe (its unlikely even in Germany people cruise faster due to economy reasons).
6th @ 140kph is about 2900rpm.
7th would be 2100rpm
8th would be 1700.

An 850CSi would still be fine at 1700RPM and would benefit from more gears. However the manual shifter might get a bit cramped.
Looking at the automatic 7speed box in the E60 M5 and you can see how the more gears allow a car with a not massive torque spread (compared to the 850CSi) but high RPM allows it to be at its sweet spot near permanently.

Every ICE has a optimum power point, a gearboxes job is to keep the engine at this point at any given speed. The more gears you have, the more successful this is going to be (see the 8speed autos on the new BMW diesels, completely changes the feel of the car to something not like a little power wedged diesel).

b318isp

Matt Simis wrote: »

The ideal number is the gear/torque reduction that gives the lowest drivable RPM at a target expected cruise speed.

The coinciding of the torque peak is the optimal point. The engine is most effective at peak torque. If that coincides with the cruise speed, then this is where peak fuel consumption will occur (all other things being equal).

Remember that many engines are not very effective at low rpm.

Matt Simis

b318isp wrote: »

The coinciding of the torque peak is the optimal point. The engine is most effective at peak torque. If that coincides with the cruise speed, then this is where peak fuel consumption will occur (all other things being equal).

Remember that many engines are not very effective at low rpm.

I would disagree when we are solely looking at fuel consumption.
Effectiveness depends what you are measuring. Peak torque is at 4000rpm in an 850CSi however its pretty clear that at 2000rpm its going to use less fuel as you do not need anything even remotely close to peak torque to keep momentum.

The point of peak torque is not relevant here, if it was then the traditional 6th gear wouldnt be a torque reducer, you wouldnt need it you would just run it in 5th at 1:1 then stop at whatever point.

Anan1

Matt Simis wrote: »

I think you are making out this is more PR and marketing than a technical requirement, I see it the opposite.

No, i'm not. Weight penalty aside, I can see that more gears = better performance/fuel economy. My point is that, beyond a certain number, it's less enjoyable for the driver. I'd happily pay a penalty in fuel consumption for the wider power band that would render 5 forward gears sufficient.

TestTransmission

6th is great, makes more motorway cruising a lot more tolerable.

Matt Simis

Anan1 wrote: »

No, i'm not. Weight penalty aside, I can see that more gears = better performance/fuel economy. My point is that, beyond a certain number, it's less enjoyable for the driver. I'd happily pay a penalty in fuel consumption for the wider power band that would render 5 forward gears sufficient.

But the power band is a function of the engine and not the gears! Its possible to put in loads of gears in terrible positions for sure, but thats not normal or what they are for.


1st gear (4.254 : 1) 2nd gear (2.534 : 1) 3rd gear (1.682 : 1) 4th gear (1.235 : 1) 5th gear (1.000 : 1) 6th gear (0.831 : 1)

has the same power band (outside of cruising gear, which the 5 doesnt even have) as:

1st gear (4.254 : 1) 2nd gear (2.534 : 1) 3rd gear (1.682 : 1) 4th gear (1.235 : 1) 5th gear (1.000 : 1)


I dont think there is any meaningful weight difference between a good 5speed box and a 6speed.
I have a 4speed Auto, 5speed manual and 2x 6speed Manuals. The 6sp cars are the most powerful and therefore need less gear changes however the idea of cutting the overdrive gears off for no gain at all (Id just loose 0.83 6th), I just cannot understand.

Anan1

Matt Simis wrote: »

But the power band is a function of the engine and not the gears.

Of course it is - the wider the useful power band, the fewer gears will suffice.

Matt Simis wrote: »

I dont think there is any meaningful weight difference between a good 5speed box and a 6speed.

Probably not

Francie Barrett

I have moved from a 5 speed Passat TDI to a newer 6 speed model and if there is any difference in economy, I certainly haven't noticed. It does cruise a little bit more comfortably when you go at 140km/hr though.

Dartz

Depends on too many things to count, really.

Going on gentle downhills I'm sometimes running in 5th or 6th at 50kph. The engine's just above idle and I don't even have to touch the throttle. It's giving 0.0L / 100km. Ging downhill is great.

Loading the engine at low RPM's is only going to increase the fuel consumptions because it's using a lot of fuel to struggle to accelerate, but at little to no load it works.

Normally, I hold on to 4th until about 70, fifth until somewhere above 80 and sixth until the end. 120 puts it at about 2300 or so....

I don't really pay attention to the econometer anyway. It lies horribly. I just let the engine run how it feels and it's usually pretty happy with that.

gollywog

I only ever used 6th in the a4 at 120kph+ anythin less was only ticking over. And had loads of pull in sixth

Thatnastyboy

I could do with a 6th gear,

close to 3000rpm at 110kph

in a 3.0 diesel

Jimbob 83

I only use 6th when cruising at 40-60mph 5th is more economical which means its a raging alcoholic as opposed to ragnarok

eth0

Anan1 wrote: »

Quite apart from any issues of wear, am I the only one who finds 6 speed manuals (and the narrow power bands that usually necessitate them) a PITA? It's like rowing the car along.

8 spd manuals ftw.

Wouldn't mind one of these. Wouldn't bother with a diesel though

Anan1

There are only seven on that.

ardmacha

There are only seven on that.

Perhaps there are two 4s.

Voodoomelon

^^ laughed out loud at that, very good!

b318isp

Matt Simis wrote: »

I would disagree when we are solely looking at fuel consumption.
Effectiveness depends what you are measuring. Peak torque is at 4000rpm in an 850CSi however its pretty clear that at 2000rpm its going to use less fuel as you do not need anything even remotely close to peak torque to keep momentum.

The point of peak torque is not relevant here, if it was then the traditional 6th gear wouldnt be a torque reducer, you wouldnt need it you would just run it in 5th at 1:1 then stop at whatever point.

Yes, but at 4000rpm, you will also be travelling twice as fast (and I'm ignoring drag/friction losses/etc.).

If you consider drag, it increases with speed to the power of 4, so becomes a major factor as speeds are higher - this explains why you are better off running at a lower rpm. It's reducing the speed that outweighs the reduced effectiveness of the engine.

Torque is important, as ideally you would want to have the engine creating the greatest output for the least fuel. CVT transmissions try to utilise the principle of keeping the engine near peak torque while accelerating. However this is complicated by the fact that only the partial output of the engine is needed when cruising, and at part load the efficiency of the engine may well be different, as you suggest.

Del2005

I've a 5 speed auto petrol with an electronic overdrive. On flat roads it goes in to overdrive at ~60km/h, I can see the tacho dropping a couple of hundred revs.

As for labouring the engine and damaging the DMF at low revs, serves you right for using tractor fuel:D and not buying auto to let the computer sort it out.

JJJJNR

I wish i knew more about the power bands and torques in my own car to make a contribution. So from my own limited point of view find 4th to be very flexible around the 80 - 100 speed especially as some of the roads along my route have very long slight inclines to fairly steep inclines, 5th is the pick of the bunch but just for the purpose of experimentation 4th seems to be a bit better at 90 and surprisingly quiet also, anyway going to see how the economy pans out over the next week.