road speed,revs and gears

[tonyh1950]

i am about to do a retest for Rospa and am doing it in a new car (not new as in new but just new to me). It is an Audi A6 TDI. I was driving it and playing with different gears and found that in2nd at 2000rpm I was doing 20mph, in third gear at 2000rpm I was doing 30mph and in 4th at 2000rpm I was doing 40mph. I haven't tried 5th or 6th gear yet.
Is this just excellent engineering by Audi or is it true of other makes as the only other vehicle I own is a saab 9000 2.3 aero which does not conform to this relationship re gear/revs.
Is 2000rpm an optimum rev for a diesel engine. I know that diesels have a higher torque and can be driven in higher gears with lower revs but this is the first diesel I have owned.

[IVORTHE DRIVER]

Hi Tony,

You will find most diesals have a much different torque curve to your Saab petrol, (you probably know that already) running from sometimes as low as 900 revs the curve will normally flatten out around 3,500/4.5000 going beyond that does not usually achieve much more in the way of acceleration.

Running up to around 2000 revs should give you good economy/ power balance (this is based on my transit ) and is also quite "mechanically sympathetic" as diesals are not known for liking high revs plus with the modern turbo motors you really have no need to go higher up the rev range.

Play and enjoy and wait for the rest of the guys to tell you something different

[jont]

a) Find a quiet bit of road and have a play in different gears at similar speeds and see what feels comfortable/flexible (eg 2nd/3rd at 20, 3rd, 4th, maybe even 5th at 30 etc)
b) RT(F)M for peak torque figures.
There's an example of a power curve for the 2l 170 TDI engine here by way of example. YMMV with a different engine:
http://www.superchips.co.uk/curves/vag20tdi170.pdf

Modern TDs might have a high fraction of torque available from as little as 1500rpm and the gears are often arranged such that you can change up at only say 3000rpm and still be at peak torque in the next gear.

[ScoobyChris]

As Jon says, find a quiet road and have a play. Imho, sitting at 2k rpm may get a bit wearing after a while and if the next gear up offers a drop in revs without sacrificing flexibility, that might be a better bet...

Let us know how you get on

Chris

[waremark]

i am about to do a retest for Rospa and am doing it in a new car (not new as in new but just new to me). It is an Audi A6 TDI. I was driving it and playing with different gears and found that in2nd at 2000rpm I was doing 20mph, in third gear at 2000rpm I was doing 30mph and in 4th at 2000rpm I was doing 40mph. I haven't tried 5th or 6th gear yet.
Is this just excellent engineering by Audi or is it true of other makes as the only other vehicle I own is a saab 9000 2.3 aero which does not conform to this relationship re gear/revs.

I would say that it is pure luck, and certainly would not be standard across different models. I also think it is slightly surprising - this sounds as though the gaps between the gears are all the same size, whereas generally the gears are closer together as you go up through the box.

By the way, remember that Audi will have designed the gearbox in a kph country, so the tidy numbers were certainly not deliberate!

I agree with most of the things others have said; play, you may well find the car is pleasant, economical and flexible at less than 2,000 rpm.

[apple tango]

I own a petrol car and it has similar, almost linear, characteristics. 2500 revs in 2nd = 20mph, in 3rd = 30mph, in 4th = 40mph and in 5th = 50mph. In any gear, adding 10mph will add approx 500rpm (so 60mph in 5th would be 3000rpm).

It's easy to remember and I've found it quite useful for making sustained gear changes smoothly.

I haven't noticed this consistency in other cars though, but then they were new to me and gear ratios were a low priority at the time so I may not have noticed anyway.

[YorkshireJumbo]

No idea if this was or is common practice, but my driving instructor in the 70s said (as a rule of thumb) you should use 1st to 10mph, 2nd to 20mph, 3rd to 30mph and 4th thereafter - this was in a Mini. It was only to show proper use of the gears in the test, so obviously you would do something different out of town. He was an enthusiast, and would regale me of his rally exploits in an imp-based special...

Was that use of the gears peculiar to my instructor, or was (and is) it more general?

[kfae8959]

In any gear, adding 10mph will add approx 500rpm

Is that really what you meant? Different gears should provide different ratios of road speed to engine speed!

David

[apple tango]

Perhaps I should quantify what I meant - it certainly wouldn't be 500 rpm per 10mph in all gears across all speeds. In the lower gears especially their range can be quite narrow.

It was merely an observation that in my car there is around 10mph difference between 2000 and 2500 rpm, and another 10mph difference between 2500 and 3000 rpm in most gears (2nd to 5th). If you exceed the useful range of a gear then clearly it won't hold true any more.

If I was doing 40mph in 4th (2500 rpm) then I could confidently select 3rd by doing a sustained rev match at around 3000 rpm. I wouldn't consider changing to 2nd.

[Gareth]

If I was doing 40mph in 4th (2500 rpm) then I could confidently select 3rd by doing a sustained rev match at around 3000 rpm. I wouldn't consider changing to 2nd.

But could you, if you wanted to? I mean two things: can the car go at 40 mph in 2nd gear, (many can), and can you manage the gear change?

[apple tango]

If I was doing 40mph in 4th (2500 rpm) then I could confidently select 3rd by doing a sustained rev match at around 3000 rpm. I wouldn't consider changing to 2nd.

But could you, if you wanted to? I mean two things: can the car go at 40 mph in 2nd gear, (many can), and can you manage the gear change?

I don't think there would be any advantage in doing that - 3rd would give me extra flexibility and power. If I went for 2nd then it would already be near the top of its power band and would therefore give less of both.

But in answer to your questions, I'm not sure to be honest - I think it would do 40mph in 2nd. I have probably reached that speed when pulling out onto a dual carriageway and wanted to get moving quickly, for example, but I do not remember ever changing down to that gear at that speed before. I don't know what the revs would be at that speed, but if I did need to change to 2nd at 40pmh I would probably go somewhere between 3500-4000.

[Gareth]

I don't think there would be any advantage in doing that - 3rd would give me extra flexibility and power. If I went for 2nd then it would already be near the top of its power band and would therefore give less of both.

Imagine you are approaching the top of a steep hill at about 40 mph, and you don't need to go any slower to go down - you might decide that changing to 2nd before starting the descent would be enough to stop it gaining speed as you started to go downhill.

[dth]

No idea if this was or is common practice, but my driving instructor in the 70s said (as a rule of thumb) you should use 1st to 10mph, 2nd to 20mph, 3rd to 30mph and 4th thereafter - this was in a Mini. It was only to show proper use of the gears in the test, so obviously you would do something different out of town. He was an enthusiast, and would regale me of his rally exploits in an imp-based special...

Was that use of the gears peculiar to my instructor, or was (and is) it more general?

This is still a useful tool in the box of speed control but should be used as guidance only. It's still used in novice instruction quite a lot because it gives some simple rules by which to choose gear selections when experience simply isn't available. In that context it is still valid.

Once experience is achieved, however, the driver should increasingly be able to respond to road conditions and general environment issues in order to continue to use the rule as a guide but to adapt it sensibly for the purposes of speed control, safety and awareness as well as economy.

The problem with many experienced drivers is that they learn this sort of stuff in the early days of driving and retain the habit rather than adapting as experience and more knowledge and confidence kicks in.

[fungus]

I had a phone call from a concerned father whose son had been given a mock test by his instructor who had failed him for not remaining in a high gear when braking towards a junction where he would be turning off, even though the car started to judder. The instructor had told the lad that he would fail his driving test if he changed down into intermediate gears. This is simply not the case. Whilst it is preferred to block change, it's never been the case that a candidate would fail the DSA test for changing down progressively. He was also told that he should be changing from 2nd to 5th when changing up, and not drive in 3rd around town but use 5th, which, as the father said, meant that the car had no flexibility at all.

I can only think that the instructor was very eco minded, and didn't want the engine to rev above 2000rpm.

[dth]

I had a phone call from a concerned father whose son had been given a mock test by his instructor who had failed him for not remaining in a high gear when braking towards a junction where he would be turning off, even though the car started to judder. The instructor had told the lad that he would fail his driving test if he changed down into intermediate gears. This is simply not the case. Whilst it is preferred to block change, it's never been the case that a candidate would fail the DSA test for changing down progressively. He was also told that he should be changing from 2nd to 5th when changing up, and not drive in 3rd around town but use 5th, which, as the father said, meant that the car had no flexibility at all.

I can only think that the instructor was very eco minded, and didn't want the engine to rev above 2000rpm.

My opinion is that the instructor doesn't know what he's doing!! BUT it's only the lad's version of what he's been told that we have heard.