Is 60mph at 1950rpm more fuel efficient than 60mph at 2200rpm?

[wemorgan]

In a manual gearbox it's relatively cheap/easy to replace the 5th gear ratio. So my question is, would driving at 60mph at 250rpm lower revs be significantly more fuel efficient?

I initially thought it would be, but then on further reading it sounds as if the only savings are from the frictional losses in the engine, plus the added benefit of a quieter cabin.

I'd be interested to read any opinions.

Thanks.

 

[lynall]

Bet the difference would be so small as to be unoticable, plus the fuel saved if any would never cover the cost of the work involved.



Lynall

 

[wemorgan]

The cost aspect is very valid. Though if I go ahead and replace the 5th gear I wouldn't justify it on cost grounds. After all many people personalise their cars in expensive ways, this would just be mine, if rather subtle.

The cost is roughly £100-£150 for the gear and ~1.5hrs labour. Not too bad I feel.

 

[oldcro]

With car being driven as opposed to on overrun at 70mph and engine doing 250rpm less, more energy would be needed. This would mean more fuel being injected which could negate any fuel efficiency. That's my thoughts on the matter but no doubt someone who knows about such things will correct my theory.

 

[Adam230K]

Changing the 5th gear ratio of a manual g/box is easy and cheap? please elaborate i've never heard of this?

 

[Will]

Maybe stick slightly bigger tyres on instead

Will

 

[wemorgan]

Changing the 5th gear ratio of a manual g/box is easy and cheap? please elaborate i've never heard of this?

The thread is super long, but here's the most relevant post. There are some others where garages have also quote 1.5hrs labour.
Audi A2 Owners' Club - View Single Post - Group buy / order: 0.65 / 0.643 5th for TDI

Maybe stick slightly bigger tyres on instead

err, you Sir are not helping! ( I assume the larger, heavier wheels would reduce efficiency, or have i got this wrong?)

 

[timskemp]

I initially thought it would be, but then on further reading it sounds as if the only savings are from the frictional losses in the engine

I'd agree with that. Are you thinking of this for the A2? Isn't 60MPG good enough?!

 

[wemorgan]

I'd agree with that. Are you thinking of this for the A2? Isn't 60MPG good enough?!

Yes, I'm thinking for the A2, but wanted to keep the thread generic enough to be relevant for MB owners

I had 72mpg the other week, so 80mpg is my now new target.

 

[LTD]

Surely the torque figures would come into play ? Would your engine require a shedload of fuel to lift it from the lower RPM if needed ?

 

[timskemp]

Yes, I'm thinking for the A2, but wanted to keep the thread generic enough to be relevant for MB owners

I had 72mpg the other week, so 80mpg is my now new target.

You ARE the anti-me!

I hold the dubious acheivement of running a 1.0 Corsa from Gatwick to Hull and averaging under 30mpg.

The problem with the A2 is finding where to tweak - how about some extreme lightening (can you lose the rear seats? Spare wheel? sound deadening), go to thinner tyres, stick a load of balloons full of helium in the back and lower it a bit?

 

[wemorgan]

Surely the torque figures would come into play ? Would your engine require a shedload of fuel to lift it from the lower RPM if needed ?

Well there begins thread no.2. The diesel remap

The problem with the A2 is finding where to tweak - how about some extreme lightening (can you lose the rear seats? Spare wheel? sound deadening), go to thinner tyres, stick a load of balloons full of helium in the back and lower it a bit?

The two rear seats are removable.
No spare wheel, just a can of gunk and compressor.
Lowering....an option, but will wait for springs to wear/crack before I replace them.

I'm mild in my obsession compared to some, who achieve 90mpg.

 

[yachtman]

Interesting thought, going on truck engines where the lowest specific fuelconsumption is at peak torque, or the slowest speed of max torque with flat torque "curves",the result will depend onwhere 60mph presently rests on the engine output curve

 

[moonloops]

If these folks are foaming at the mouth to save a few pence on fuel then they should just walk, meanwhile I'll enjoy hoofing around in my V8. I don't do mega miles so it's justified

 

[flango]

I'd have to do the maths and see where it sits on the torque curve but off the top of my head I think the difference would be minimal or non existant for 250 rpm less

 

[wemorgan]

I'd have to do the maths and see where it sits on the torque curve but off the top of my head I think the difference would be minimal or non existant for 250 rpm less

Before a remap peak torque ~150 lbft, 2000-2500 rpm.
After a remap peak torque ~200 lbft, 2000-2200 rpm, but there's 180 lbft 1800-3000 rpm.

 

[scumbag]

It must save some fuel.

you would only be using. lets say it was a 6.3v8 at 1 rpm. using 6.3ltrs or air per revolution and sparking or igniting 8 times. so at 2200 it using 2200 ltrs or air per minute and sparking 8 times per rev

reduce that to 1950 and you reduce everything.

Including fuel consumption.

However, without a remap (To suit the gearing and your new found economy drive), you are losing power and torque so the car will not be as powerful at 60 as it used to be. You may find you need to put a bit more pressure on the pedal, which in turn increase the fuel flow to keep your car moving at 60mph.

This is due to the air being moved by your car at 60mph which is significant. You can work it out. frontal area of your car, times forward momentum. Your CD figures will show you at what point the air is the greater resistance than the wieght and frictional loses of your car and you end up knowing when your car is working, rather than rolling. Safe to assume it is around 37mph ish.

so you are inceasing the workload your car engine is doing at 1950rpm.

what you need to find out is whether the car will move that amount of resistance at 60mph. (It probably will). The issue then relates to the higher in the rev band. At 70 your car will be 250 revs less for each given increase in 1mph. Your car might require the engine to be at 2800rpm just to do 70mph, hence why it is doing 2200 at 60mph.

so at 2550rpm, you might need a firm prod on the loud pedal to keep it moving thus aplying more fuel as it is now in acceleration rather than cruise as it were. The power is being applied to maintain the current speed.

You will no doubt continue to lose out the higher speed you go, and the top speed will be reduced as the car will just not produce the power it needs to acheive it, assuming your car can reach its top speed in top gear.

None of this assumes the wheel to rev ratio which again will effect it. The larger the engine and/or the lower it generates torque, the more chance you have of seeing a fuel saving at these speeds.

And don't forget it won't be able to accept the gear change until you get to a higher speed which may remove the potential saving.

ie, change up to 5th at 30mph, will now be 33mph. if you haven't altered 4th, your car will need to run at a number of rpm more than currently just so the car will actually move in 5th. you may add 300rpm to it when changing from 4th to 5th, 30 to 33mph as it were, which will remove all your saving and add 50rpm.

This is not a scientific set of fiqures above. just the obvious points that need to be thought of.

You will need to do some digging about to see whether it will actually make a diffence.


VW's polo blue motion has a higher top gear, along with lower rolling resistance tyres, better aero body and reduced power engine, and it achieves the desired goal. Fords stuff a larger deisel in their fiesta for the eco range, just to keep it moving at slow speeds at a better lick then the oppositions do as well as it can use lower revs at 60 or so for better consumtion.

Both will have a significantly reduced top speed even the the cars have better aero and lower resistance tyres. The savings are a combination of parts not the increase in gears.

You could probably save the same amount of fuel with a custom remap to have the throttle use less fuel for a given set of revs, but would increase the amount for the higher revs, so you could keep the same speed, use less fuel, and still access the power of your engine when needed (but would require a drop of gear and more revs to access it)

maybe.

you will probably save a 0.09% of fuel by doing this, but it would be pretty difficult to record. You might get a similar saving using lower friction oils, or increasing the tyre pressures to reduce rolling resistance.

 

[wemorgan]

Some interesting points there. Thanks.

I wondered if when driving at constant speed (which I do a lot) the cars needs say 20bhp to hold 60mph, whether with/without remap or any gear ratio changes that the energy and therefore bhp required will actually change.

 

[scumbag]

I wondered if when driving at constant speed (which I do a lot) the cars needs say 20bhp to hold 60mph, whether with/without remap or any gear ratio changes that the energy and therefore bhp required will actually change.

To be honest, I have forgot the in's and out's of the way you work it all out. Probably due to them being particulalry complicated calculations only of any use for that particular need, and me not having any need for them.

(Currently looking at nozzle discharge rates, and height and speed for setting an application rate to apply anti icing agent to 66,000sq m of ashpalt. need 25grms pr sq m, SG 1.28)

There was a sheet with the proportion of air to speed ratio kicking about on this site somewhere which would give some starting point on answering that. Probably the 56mph thread.

Then you need to work out the weight of your car and the power required just to move that. this should include the frictional resistance of the tyre. all of which increase with speed. Your engine power graph, if you have one, should give you an idea. (You may have one in the orignal manufacturers brochure but potentially that it it's design output and the car could be a bit less, or more, then age to take into considertation, some horses will have bolted) and you will now know your car's output at that given engine speed.

As far as the original question is concerned. if you need lets say 20bhp for 60mph, and your car is doing 38bhp at those revs, and 33 at 250rpm less. you should save fuel as it is producing more than it needs. In fact, its not running as effeciently as it could, so you could probably increase the speed to equal the 33bhp for the same fuel consumption and thus go further. ie increase the mpg. If you lower the power to meet the exact at 60mph, you will lose out at 61 plus.

Of course, if you fill the car with people, it might need 34bhp to do 60mph.

That said, I am sure you need more than 20bhp to shift 1.5tonnes at 60mph.

because your car will accelerate reasonable at 60mph, it is a fair chance to think it will be producing more power than it needs, and could probably drive almost as well by 250rpm reduction. But the effects will be increased proportional to the increase in speed.

They have slowed most efficient aircraft down to save fuel. The reason is that they have an efficiency operating window in which you can set the engine to power ratio for its most effective, but as they chase 1 or 2% fuel reductions, you can see this by reducing 70 or so knots of airspeed, adding about 25mins to the time for a trans atlantic flight and 1% of 170,000ltrs at 60p a ltr is a pretty big saving.

your reciprocating engine is not as efficient, and you are only taking 250rpm out of it, so again you are likley to see a very small reduction in fuel if everything was in your favour. You may even see the same increase in fuel efficienty by using better fuel or having your remap to suit 98ron. Entirely financially cost ineffective as it costs 4-8% more, but you could see a 2 or 3% better mpg. Now add that to a 250rpm reduction and it might start to break even, assuming you write off the costs of doing the work.

And that's petrol. Diesel engines being much more efficient to start with, so any 1% increase will be even less noticable.

To downside will be depening on the engine being used to do this. If it already a fuel efficient set up you may find you are on the edge of the performance line as it is, in which case, its all down hill. Or you may take out the builders in built comfort margin, which means you will not see anything, as you can't measure it to any set of specs.

So lots of rolling road sessions, increasing tyre pressure, fuel type changes, different oils, different tyres needed all of which to get a set of baseline results from which to work.

finger in the wind, if the car is easily capable of over 100mph as it is, then you will see a fuel saving by changing the ratio. Whether you can measure it will be a good question! (easliy capable means 10% or more)

If its a FIAT Panda 4x4, forget it.


how long are you planning to take over this project? Its a very green and admirable way to lower your CO2 emmissions.

I would just walk or cycle to work one day a week to do my bit.

 

[wemorgan]

how long are you planning to take over this project? Its a very green and admirable way to lower your CO2 emmissions.

I would just walk or cycle to work one day a week to do my bit.

I guess I'll continue to have an interest for as long as I keep the car. But I seldom keep any car beyond 1 year, but I'm on my 2nd A2 now, so there must be something about them I like.

I commute 300 miles a week, so any fuel savings can add up to some difference. But even then a 10% saving is only ~£2/week, so it's all really an academic exercises rather than a real money saving idea.

If I can manage to own a practical car that was designed 10/15 years ago, that can seat 4 adults plus baggage, than can do +100mph, gives a range of 600-700 miles, by averaging 70-80mpg, it would make me wonder what OEMs are really achieving today.