Hi,
a theory question but if you have any pracitcal knowledge, that would be great.
Scenario, at 50MPH 5th gear is turning at 1750RPM, 6th is turning at 1500RPM, engine is a turbo-diesel with peak torque at 1750RPM.
What gear is the most economical, 5th at peak torque or 6th with lower RPM?
Please disregard the effects of accelerating above 50MPH in 6th gear for this question as I know the car would be a bit 'flat' this low in the RPM range.
I ask this question as I would drive in 6th at this speed unless lots of hills but some people are saying it's more economical to be in 5th at this speed due to peak torque.
What's your thoughts?
Thanks
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The most expensive fuel part of driving a vehicle is moving it from rest - once it has reached the speed required, normally the highest (suitable) gear and the minimum throttle setting needed to maintain the speed is sufficient.
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What\'s for you won\'t pass you by
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I was taught about piston engines by a superb RAF Flt Sgt. We had the Bulldog with a variable speed prop, and he taught us that low revs with high manifold air pressure (we had a gauge for MAP) for was much more economical than higher revs and lower MAP.
My father had an A40 with an aftermarket economy meter that was just an MAP guage with green in the lower end of the dial.
On the basis of what I was taught I'd suggest that 6th is the most economical gear. Not sure what the torque figure has to do with it. Max torque will require high amounts of fuel.
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Low revs and high throttle opening gives better economy than higher revs and lighter throttle. This is because the engine is running at higher Brake Mean Effective Pressure and hence better efficiency. How different? Well a typical TDI running at peak efficiency will consume about 200g/kw/h, while running at light throttle e.g. like most of our cars do all the time, it will use about 350g/kw/h... For petrols it's about the same ratio it's just peak efficiency is more like 250g/kw/h (very rough figures here). What we really need in all our cars is a gear in top that allows our engines to sit at high throttle openings and very low revs for optimum efficiency, but motoring journalists would write that car off as unresponsive at cruising speeds.
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I have a Scenic dci with 6 gears and two separate dealers have told me not to use 6th gear till 50mph .
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>>a theory question
Yes, but a theory question that is unanswerable given the limited amount of data we have.
To answer the question in a rigorous way, among other data, the engine maps, or contours of engine speed, BMEP and BSFC contours are needed.
We don't have them, and unless you already have, or can find the data published anywhere (unlikely!!), the answer remains beyond our reach.
In a practical sense, the only way to find out is if someone with a similar car has a fuel computer fitted. Although the absolute numbers displayed by this gimmick are worthless, the relative values obtained in each gear might be helpful.
In a general sense, the power required to push the car along at 50mph is constant - it doesn't matter what gear you are in (unless one of the gears is a direct drive, and you obtain some transmission efficiency gains there). Typically, the best BSFC figures are at mid load, and near to the max torque engine speed.
Number_Cruncher
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Now then, bet there isn't another site on the web, that'd give such concise & varied explanations, as those provided by brainy B'roomers!!
My input would be that it made me recall customer wanting to buy a new van, a fair few years back. There was me proclaiming the advantages of 5 speed gearboxes, as a newish development. He wasn't worried, as he didn't often use the 4 he already had!! Old joke, I know, but this time it REALLY happened!!
VB
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My car has 6 gears, the only trouble is the 6th is reverse!
--
Roger
A fine is a tax for doing wrong. A tax is a fine for doing well.
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I don't use 6th gear on my Mondeo ST TDCi until I'm cruising at 60mph or above, and I'll drop it into 5th if the speed drops below 55mph. It will pull from those sorts of speeds but it just feels a touch laboured and engine vibration on a modern TD is putting the dual mass flywheel under stress.
If I'm accelerating quite hard, eg. when joining the motorway I frequently use 4th upto 70mph and go straight to 6th.
To answer your question directly, on the flat there would be a very slight improvement in economy by using 6th gear on the flat. However, if you need to increase throttle either to maintain 50mph due to a gradient or to increase speed, you would be better remaining in 5th.
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I have a Focus 2.0tdci and am fairly confident that my fuel computer would say that at 50-60mph, 5th is more economical than 6th. I have no idea why but am fairly sure that's what it would say based on past experience. 6th is for 70+.
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The instruction book for my 1949 Ferguson tractor says you know you are in the right gear if it accelerates when you open the throttle further. If it doesn't, it's in too high a gear.
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>>for my 1949 Ferguson tractor>>
Do you suffer from plough-on understeer?
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What\'s for you won\'t pass you by
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6th turning at 1500RPM is the more economical as long as it's on the flat or decline. If going up hill then use 5th gear to save engine stress and use more torgue.
In general there is a trade off between an engines peak torque and torque characteristics verses things such as rolling resistance, wind drag, friction, weight of vehicle, gear ratios etc.
Some cars have a 6th gear that has the same (or similar) gear ratio as another cars 5th gear, for example Megane and Focus. Some it to is to do with marketing and selling the cars and some is to do with providing more choice of gearing for high torque engines. A lorry is a good example of loads of gears to get a heavy vehicle moving from standstill whilst at the same providing drivability and economy once on the move.
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>>for my 1949 Ferguson tractor>> Do you suffer from plough-on understeer? - - - - - - - - - - - - - - - - - - - - - What\'s for you won\'t pass you by
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I daresay when muck-spreading there's a risk of skid starts.
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" Low revs and high throttle opening gives better economy than higher revs and lighter throttle."
Having a VW TDI which has no throttle or throttle cable (drive by wire) surely this is not possible, as it is all controlled from the ECU/
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I also have a VW and can ease the throttle off once the required speed is reached - the evidence of the difference is shown by the computer average mpg figures.
Remember the cars of the 60s (?) and 70s which had an overdrive button on the gearstick knob, Triumphs being amongst them?
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What\'s for you won\'t pass you by
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The old 2.5 DI Transits had a five speed gearbox (MT5 I think) but fifth gear wasn't part of the main gearbox. It was sort of welded on in a gearbox extension housing.
Everything worked fine unless fifth gear was overstressed by low speed high torque use - something it wasn't designed to deal with. Broken welds and loss of fifth gear was a common occurrence.
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The recent spate of CR failures has been attributed to running the car at too low revs for prolonged periods.
Who cares about the extra 2mpg if your turbo fails as a by-product?
none has it spot on. What you guys are talking about is the equivalent of running a petrol car in 5th at 30mph. Yes it'll work, but you won't be smiling when the engine sludges up and the big end goes after a few years.
I'm not convinced about the lower revs=more mpg argument anyway. Think of all the extra work you have to do when riding a bike slowly at high gear.
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Remember the cars of the 60s (?) and 70s which had an overdrive button on the gearstick knob Triumphs being amongst them? - - - - - - - - - - -
My 1964 Triumph 2000 has an overdrive (works in 3rd and 4th) and also my Volvo (1993). Volvo were the last manufacturer to offer overdrives, fitting them as a parallel option to the 5-speed boxes for a long time.
In my opinion they are much better than 5 or 6 speed boxes. Faster, effortless clutchless gear changes. Discontinued for economy reasons I suppose.
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Cliff, My 1964 T2000 had the overdrive lever mounted on the steering column.
Marvellous to be able to change gear with the flick of a switch.
Only problem was the solenoid mounted on the back of the box used to fail.
I think that was rectified on the 2.5.
wemyss
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