question re economy and rpm thats bugging me
Discussion
I'd guess no. Work would be the same distance away. Say 10,000 revolutions of the driven wheels.
In 5th it would take maybe 9,000 revolutions of the engine.
In 3rd it would take maybe 15,000 revolutions of the engine.
You were halfway there by thinking it's the same revs per minute, but by virtue of the fact you're going slower, you're doing the same RPM for many more minutes.
EDIT: on reading the OP again, there is an argument that if you ran the car for five minutes both in 3rd and 5th (ignoring the fact that you'd want to actually get to a destination) there's an argument that it WOULD be more conomical in 3rd as the gearing means there's be less load on the engine, so you'd need less torque (and thus fuel) to maintain your theoretical 3500RPM. But you'd have to walk the rest of the way.
The slight gains in the second situation are far outweighed by the problems in the first, by the way.
In 5th it would take maybe 9,000 revolutions of the engine.
In 3rd it would take maybe 15,000 revolutions of the engine.
You were halfway there by thinking it's the same revs per minute, but by virtue of the fact you're going slower, you're doing the same RPM for many more minutes.
EDIT: on reading the OP again, there is an argument that if you ran the car for five minutes both in 3rd and 5th (ignoring the fact that you'd want to actually get to a destination) there's an argument that it WOULD be more conomical in 3rd as the gearing means there's be less load on the engine, so you'd need less torque (and thus fuel) to maintain your theoretical 3500RPM. But you'd have to walk the rest of the way.
The slight gains in the second situation are far outweighed by the problems in the first, by the way.
Edited by crofty1984 on Monday 6th September 18:39
Depends on the car. Any fuel injected car will cutoff the fuel on the overrun so it's best to leave it in gear all the way down for economy. For a carburetted car the best thing to do for economy is to dip the clutch and slow on the brakes. For road driving the only reason to match revs is to show off. 
The Tea Boy said:
Say for instance that at 70mph in 5th gear(where my car is fairly economical) the car is doing 3750rpm, would driving at lower speeds in say 3rd gear give the same mpg?
Sorry for the random question but was thinking about it on my way to and from work.
Matt
I guess it depends onhow slow. There must be a point where the additional internal friction caused by high revs will be outweighed by the reduced aerodynamic drag.Sorry for the random question but was thinking about it on my way to and from work.
Matt
SS7
I'd say yes because the effects of drag are higher the faster you go. Thus if your revs are 3750rpm in 1st gear then you'll probably only be doing 25mph, whereas in 5th you'll be doing say, 70mph. Neglecting the gearing, the drag will be significantly higher at 70mph than at 25mph so will be less efficient.
I have to say your question is bugging me too as I'm now thinking about the respective torque outputs from the gearbox. Gears are torque multipliers so more is delivered at a lower gear. However I don't have a full understanding of automotive mechanicals so am not 100% if there is an optimum gear ratio for a given car that will deliver the engine's torque most efficiently. Logic would say 1st gear is most efficient but maybe someone more knowledgable can answer for sure.
ETA: Maybe a more simplistic way of thinking about it is to ask yourself if you could ride a bike for longer if your legs were spinning at 70rpm in 1st or 20th gear?
I have to say your question is bugging me too as I'm now thinking about the respective torque outputs from the gearbox. Gears are torque multipliers so more is delivered at a lower gear. However I don't have a full understanding of automotive mechanicals so am not 100% if there is an optimum gear ratio for a given car that will deliver the engine's torque most efficiently. Logic would say 1st gear is most efficient but maybe someone more knowledgable can answer for sure.
ETA: Maybe a more simplistic way of thinking about it is to ask yourself if you could ride a bike for longer if your legs were spinning at 70rpm in 1st or 20th gear?
Edited by Fatman2 on Monday 6th September 22:35
There will be certain matching points on a fuelling map for different load/RPM conditions.
At a lower speed you have less drag, so you'll be using less throttle to maintain RPM. If you artificially loaded the car to the same conditions and applied the same throttle then yes - it should be the same.
In summary, the gear is irrelevant.
ETA (as did everyone) the question is slightly different to the one I thought! It's too late to think about it now.
At a lower speed you have less drag, so you'll be using less throttle to maintain RPM. If you artificially loaded the car to the same conditions and applied the same throttle then yes - it should be the same.
In summary, the gear is irrelevant.
ETA (as did everyone) the question is slightly different to the one I thought! It's too late to think about it now.
Edited by Mattt on Monday 6th September 22:41
Isn't it true that at peak torque you are at the sweet spotof the engine in terms of efficiency?
However even if not true for all the cars I've had this has been the case peak torque does equal best mpg.
Therefore if they only could make a cvt box which had an Eco mode so as holding the revs at peak torque from starting to move up to the point the car doesn't have the torque to push the car any faster. Plus a sports mode I cannot see any need for more than two modes one Eco one sports job done.
However even if not true for all the cars I've had this has been the case peak torque does equal best mpg.
Therefore if they only could make a cvt box which had an Eco mode so as holding the revs at peak torque from starting to move up to the point the car doesn't have the torque to push the car any faster. Plus a sports mode I cannot see any need for more than two modes one Eco one sports job done.
Dracoro said:
If driving at "peak torque" all day was actually best for economy (it isn't) I'd be driving at 7,500rpm all day
Low revs as but not so low that the engine labours.
Regardless, top gear on the motorway is best.
lol wellLow revs as but not so low that the engine labours.
Regardless, top gear on the motorway is best.
fiat coupe 20vt peak torque 2,800 rpm bang on 70mph
330d peak torque (although it's a plateau) 1,700rpm 70mph
golf gt tdi 115 peak torque 1,900rpm bang on 70mph
mondy peak torque iirc was 2,000rpm bang on 70 mph
cavalier Sri 16v peak torque 3,500 rpm again 70mph
seems maybe car makers are designing the ratios to provide peak torque at m way speed in top gear possibly in the choice of cars I've had they are more economical. However high revs isn't the ctr @70mph 4500rpm anyway in top fear?
Two numbers - Gallons/Hour and Miles/Hour. Gph is affected by how wide the throttle is opened, Mph by how fast you are going. Peak economy for cruising will be achieved where
Miles/Hour
Gallons/Hour
Is the largest number possible. To measure it would take quite a lot of time and a test track, but generally for nearly every car this will be somewhere around 50-60mph.
The peak efficiency (energy released as mechanical force per fuel thrown in) for most cars is usually around full throttle around the peak torque revs, so to extract maximum energy from a litre of fuel, hammering it around peak torque revs is best.
In reality, you will spend very little time needing high levels of torque or power.
So for steady state cruising a small proportion of the total engine power is needed. In this case you need to study the brake specific fuel consumption graphs to find the best gear for a given power output. These graphs show the fuel consumption plotted against power output, cylinder pressure (BMEP), and revs.
In most petrol cars, the best mpg will be at high load and low revs, i.e. lots of throttle, low revs, in a high gear. Some reasons for this include low pumping losses in throttled petrols as the throttle plate is open quite a bit (but this doesn't apply for direct injection ones). Also, there are less frictional forces wasting energy due to the lower revs. Also at full throttle some petrols dump extra fuel in to prvent running too hot and melting pistons, but at low revs I don't think full throttle causes extra fuel enrichment in modern fuel injected petrols.
On Diesels there is no throttle plate causing extra pumping losses, but at low power levels, the best economy is with heavy throttle at low revs, even below the turbo coming on boost. The BSFC charts reveal this also.
Autoexpress did an economy test a year or so ago on lots of cars, petrol and diesel. The best mpg was from around 20mph to 35mph on a range of cars. From memory the Citroen 1.6HDI managed over 100mpg at 19mph, so probably in 4th gear at very low revs.
As for whether the OPs car is more effieicnet at 3750rpm in 3rd or 5th, well I would say it will be similar in most cases. In 3rd the engine won't be highly loaded, so won't be operating at a high BSFC level, but will need less power to motor along. In 5th, more power will be required to overcome drag, but the engine will be producing it more efficiently as it will be more highly loaded (i.e. less fuel needed per BHP, back to the BSFC graphs again). The better way to fuel economy would be to travel at the speed gained at 3750rpm in 3rd gear, but put the car into a higher gear at that speed, 5th gear for example = less revs, less frictional force, less pumping losses, and a more efficient burn of the fuel (more load and lower revs).
In reality, you will spend very little time needing high levels of torque or power.
So for steady state cruising a small proportion of the total engine power is needed. In this case you need to study the brake specific fuel consumption graphs to find the best gear for a given power output. These graphs show the fuel consumption plotted against power output, cylinder pressure (BMEP), and revs.
In most petrol cars, the best mpg will be at high load and low revs, i.e. lots of throttle, low revs, in a high gear. Some reasons for this include low pumping losses in throttled petrols as the throttle plate is open quite a bit (but this doesn't apply for direct injection ones). Also, there are less frictional forces wasting energy due to the lower revs. Also at full throttle some petrols dump extra fuel in to prvent running too hot and melting pistons, but at low revs I don't think full throttle causes extra fuel enrichment in modern fuel injected petrols.
On Diesels there is no throttle plate causing extra pumping losses, but at low power levels, the best economy is with heavy throttle at low revs, even below the turbo coming on boost. The BSFC charts reveal this also.
Autoexpress did an economy test a year or so ago on lots of cars, petrol and diesel. The best mpg was from around 20mph to 35mph on a range of cars. From memory the Citroen 1.6HDI managed over 100mpg at 19mph, so probably in 4th gear at very low revs.
As for whether the OPs car is more effieicnet at 3750rpm in 3rd or 5th, well I would say it will be similar in most cases. In 3rd the engine won't be highly loaded, so won't be operating at a high BSFC level, but will need less power to motor along. In 5th, more power will be required to overcome drag, but the engine will be producing it more efficiently as it will be more highly loaded (i.e. less fuel needed per BHP, back to the BSFC graphs again). The better way to fuel economy would be to travel at the speed gained at 3750rpm in 3rd gear, but put the car into a higher gear at that speed, 5th gear for example = less revs, less frictional force, less pumping losses, and a more efficient burn of the fuel (more load and lower revs).
Welshbeef said:
Dracoro said:
If driving at "peak torque" all day was actually best for economy (it isn't) I'd be driving at 7,500rpm all day
Low revs as but not so low that the engine labours.
Regardless, top gear on the motorway is best.
lol wellLow revs as but not so low that the engine labours.
Regardless, top gear on the motorway is best.
fiat coupe 20vt peak torque 2,800 rpm bang on 70mph
330d peak torque (although it's a plateau) 1,700rpm 70mph
golf gt tdi 115 peak torque 1,900rpm bang on 70mph
mondy peak torque iirc was 2,000rpm bang on 70 mph
cavalier Sri 16v peak torque 3,500 rpm again 70mph
seems maybe car makers are designing the ratios to provide peak torque at m way speed in top gear possibly in the choice of cars I've had they are more economical. However high revs isn't the ctr @70mph 4500rpm anyway in top fear?
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