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A few days ago somebody posted a question concerning what was the best rpm for Max fuel consumption? I replied that, so far as I was aware, it was the rpm at which max torque is developed and added that this might be hard to find out. I have since found that the torque figure and the rpm at which it occurs is printed the New Cars Tech Spec section of Wh*t Car each month.
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The revs where max torque occurs will indeed give the most efficient SPECIFIC fuel consumption but reducing the revs below this will give better ACTUAL fuel consumption.
For example if a car gives max torque at 3,500 rpm and top gear is 20 mph/1000rpm, then 70 mph will give the best specific fuel consumption, but reducing to 60mph / 3000rpm will give better actual fuel consumption because the power/torque required is about 25% less.
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I've got a friend who changes gear very early (always in 5th by 30.) She tells me off for changing at about 3000 (309 1.4 car, she has same) - both cars pull better and need less throttle to maintain a steady acceleration if changing at 3000-ish. Am I right in saying I'll get the better economy? She also got annoyed with me for once running the carv through to its limiter joining a motorway in 2nd/3rd; I said I very occasionally do this because I've heard/read that it can be good for the fuel injection (hers is a carb engine btw so I don't know if the same applies.) Incidentally hers is burning oil.
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She may save a little fuel (although possibly not, as the slower acceleration will mean that she will be in the intermediate gears for longer) and will certainly wear the bottom end of her engine out sooner. Does she hang her handbag on the choke, too? (Sorry - am standing in the corner already).
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She's a good driver (!) but changes gears at no more than say 2-2500 most of the time, but still tries to accelerate quickly 'my engine doesn't like revs as much as yours.' Her foot stamps down on the accelerator after every gear change. As I say she is a good driver but its a little argument we have! I drove her car and like mine it needs much less throttle if revved to 3000ish. Hardly ever uses the choke as she has it warmed up in about 30 secs...
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Dave -
Yes, I have encountered this \"must get it in top gear\" approach -and not just from elderly people.
If the \"5th gear at 30\" trick was done on a deserted airfield, there would be no problem.
BUT in the real world, the negotiating of hazards in too high a gear reduces SAFETY significantly.
There is no flexible control of road speed and nothing in reserve - no power ready for the unexpected.
As an approx. guide, (obviously non-vehicle specific) I teach 20-40 in 2nd, 30-50 in 3rd, 40-80 in 4th and 50-100 in 5th - this may not be exactly right for your car, but is simple to remember.
(A by-product of this is to reduce the \"sheep\" driving at 40 in 30 limits)
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Trevor's point is rather more valid than considering the absolute rev/ speed combination at which an engine is most efficient. In the real world we are making constant adjustments to speed sometimes these are almost infinitesimal, and it is much easier to do this if one is in an appropriate gear. This can mean with an autobox selecting something other than D around town or on a twisty road, for example, as it makes the whole thing more balanced and controllable.
Also whilst noting the caveats around Trevor's method of teaching appropriate gear/speed ranges am I right in thinking that this is an easier way of getting a result with a pupil than perhaps getting them to understand the "engineering" behind it all which would enable the pupil to apply the "method" in an unfamiliar vehicle and thus have mechanical sympathy? Convoluted question I realise but you know what I mean.
As you say Trevor it varies car to car, for my own diesel 3rd is a great around town gear and would say the typical range is low 20's to ~50. Funnily enough 3rd is also a fine gear for blatting along twisty roads and is still reasonably in the torque band at 60.
Also I found it interesting that the engine of Dave18's friend is burning oil. I wonder if this is due to bore glazing, especially if it has been driven like this from new.
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FiF -
Good guess -
Any suggestions about segment(s) of the driving population that are not too keen on "engineering behind it all which would enable the pupil to have mechanical sympathy?" ?
Spot on also re gear selection in Auto.
For my Auto demo drive, I have a route which includes a school followed by a steep downhill 30.
I change down (using controls on s/wheel if fitted - brilliant!) to 2nd gear on approach to school.
You should see the dirty/are you mad? looks I get.
So what do the non-thinkers do? Go all down the hill in D, braking all the time doing about 40.
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Although I agree with your comments about being in too high a gear for the road conditions being dangerous on occasion I have found that being in 4th by 20mph and then up to 5th at 30mph considerably improves my mpg, and the engine pulls adequately enough from these speeds in 'normal' driving conditions (also because thrashing it seems pointless as its overall performance is poor).
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What car do you drive? Is it an 8 valve engine? I understand they have better low down torque than 16 valvers.
My Xsara 1.4 pulls from 30 in 5th without any fuss. I would hate to have a car that I couldn't use 5th until 40 or 50mph. As a bonus, it is really easy to get 40mpg when not sitting still in traffic jams.
Ben
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Bennyboy, I have exactly the same engine as you, but mine is in a Citroen ZX. Its low gearing helps make it quick from 0-40 but the performance falls off rapidly above this (it hates being in 3rd above 50mph!)
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Gentlemen you are totally missing the point here. Engage brains please!
You are only considering the ability to maintain a constant speed and accelerate, albeit slowly, from 30 in 5th. What about another important aspect of this issue, namely using engine compression to slow down?
At 30 in 5th my engine is already around its tickover speed, not desirable from either engineering or safety viewpoints in my humble opinion.
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FiF -
I think you are wasting your time.
The "I do this" and "my car is wonderful" and "Speed cameras are terrible - because they stop me driving at REASONABLE speeds"
are all variations on the same "non-thinking" theme.
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I find that brakes are the most effective method of slowing down all the cars I have driven. That is what they are for. The engine is for maintaining and increasing speed.
The need to be in the correct gear at the correct time is for two reasons. Firstly to maintain reasonable progress. What one person defines as reasonable may well differ from others. Secondly to realise acceptable levels of fuel economy and minimise wear on mechanical parts. Again definitions of acceptable will vary.
One of my cars is quite happy accellerating at 30 in 5th, the other is barely happy in 4th at that speed.
Andy
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Oops, sorry, missed a point. Apart from long downhills!!
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I find that brakes are the most effective method of slowing down all the cars I have driven. That is what they are for. The engine is for maintaining and increasing speed.
Obviously when you were learning to drive then, you missed the lesson on engine braking? It is a proven fact that you will save fuel by using the gears for de-acceleration as well as acceleration.
Are you the bloke I follow to work most mornings who is forever dabbing on his brakes rather that being in the appropriate gear and using throttle control?
That pedal to the right isn't an on/off switch you know :o)
You don't drive a Volvo do you :o)
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I was told by my instructor to use the breaks to stop rather than the engine (not that I head his advice all the time) as it is cheaper to replace breaks than a gearbox (due to increased use). Not sure how much more stress engine braking puts on a box.
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It depends how you change gear, the box will more than cope with any amount of acceleration/deceleration, but:
If you match the engine speed and road speed when changing down virtually no wear will occur in the clutch or syncro.
If you slam the gear change through with the revs unmatched you will prematurely wear out both the gearbox and the clutch.
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Obviously when you were learning to drive then, you missed the lesson on engine braking?
The only mention of engine braking that I recall in driver training to the basic test standard was the one that is can be helpful to descend hills in low gears to avoid overheating brakes. However, I will certainly admit that my memory so far back is hazy at times.
It is a proven fact that youwill save fuel by using the gears for de-acceleration as well as acceleration.
Rubbish - there is no way on earth that any difference is made to fuel consumption whether you use engine, brakes or parachute to slow you down. Unless fuel is not cut off on the overrun, in which case engine braking will use more fuel. Avoiding the need to accellerate by looking ahead and anticipating the traffic flow ahead certainly can save fuel, however.
Are you the bloke I follow to work most mornings who is forever dabbing on his brakes rather that being in the appropriate gear and using throttle control?
Certainly not. I'll be the one hardly using my brakes, but hanging back and trying to minimise braking and the consequent need to accellerate.
That pedal to the right isn't an on/off switch you know :o)
And the point of that statement is what?
You don't drive a Volvo do you :o)
Again, certainly not. Although I do quite fancy a T5!
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>> That pedal to the right isn't an on/off switch you know >> :o) And the point of that statement is what?
You failed to spot the smiley :o)
The point I was trying to make (but not specifically at you) was that some people accelerate and brake for what seems like no reason at all, thus treating the loud pedal as a switch instead of a variable means of control of the engine.
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I've been a passenger in a car driven by someone who drove that way.
Not a happy experience!
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Calm, gentlemen.
And if you want to continue the discussion about driving techniques and tests etc. then may I suggest you take it across to the Discussion Forum.
Mark.
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While i agree with your comments regarding a peak torque of 3.5k, this is not practical for my 1.8T, max torque arrives at 2k through to 4.5k,driving down to 1.5k is possible on the flat with slight throttle opening, but there is no real power available this low down much better to drop a gear and run at just over 2k.
Also regardless of where the max torque arrives greater speed in a given gear will always increase consumption.
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