Fuel Consumption - 60kph Vs 70kph

[Ives]

I was just wondering which one would use more fuel and why? :-

1) Travelling 60kph at 2000rpm
2) Travelling 70kph at 2000rpm

Speed is the only variable...car, driver, conditions etc of both scenario are identical.

[james22]

I would suspect 70 kph, even though the engine is under the same load (2000 rpm), the wind resistance at 70kph would be increased over 60kph, so theoretically you would think 70 kph would use more...

[micksta]

but u'd also cover a greater distance .. so would going faster at the same revs make u travel more or the same ?

[bArNsY]

Assuming they are traveling at a constant speed
I would think it depends on how much one would be pressing the accelerator down!

[micksta]

i reckon mythbusters should do a segment on this.

[poolkeeper]

I think about this too, whether I should go slow 90kph or go 120 and keep up with traffic.
I think its the way you get to a higher speed, that determines more fuel usage...

[The Monty]

Did noone learn from the plane on the treadmill thread? lol, Sorry to go off topic, but thats a good question.

[Powdered Toast Man]

How is speed the only variable? Something else must be different otherwise you couldnt do different speeds at the same rpm, whether its gearing or load.

If there truely was no variable other than speed than theoretically 70kmh would be more efficient because you are travelling further for every revolution of the engine. Therefore your fuel consumption (litres per hundred) would be lower.

[Joes_meat]

They say for most cars 80kmph in top gear is the most fuel-efficient speed to travel at.

For cars with large engines and low gearing, it's more like 90-100kmph.

70kmh will most likely use more.

[HLC]

i once read that something like 80 kays per hour is twice as economical as 120 kph. in terms of tyre life/wear.

but i could be wrong.

[micksta]

speed wouldn't be the only variable .. like james22 said .. wind resistance.

but u would think that with a head wind .. both going at 2000rpm .. the one going faster would have the throttle slighter pressed a bit more .. using more air and fuel to keep the momentum going.

thats my understanding .. but its not what i think on the roads though.

getting there faster = less fuel used lol.

[Yellow_Festiva]

Mythbusters have already did this, they used a Lotus super 7 replica (clubman i think with the toyota 1.6 20v engine)

Now they thrashed it, then drove reasonably paced then drove like a granny. Granny mode was the winner over fuel consumption.

I have tested this extensivly with the Festiva, and the way to get maximum km/l is to drive the car in the highest gear possible at the slowest possible speed before you start to labour the engine. So that means i get best results driving in 5th at 55-60km/h

I bought the festiva new, and when running in didnt get over 80km/h. I was AMAZED at how good on juice she was, even bettering the factory claims at times.

Ran her in, started to drive like normal and consumption increased by 20%.

And also driving to uni, mostly express way driving, the faster you go the more juice you use, however i tested the fact that if i went at 80km/h i would get say 550km to a tank, then if i went 100km/h i would get 500km to the tank, but if i kept going at 110-115km/h the range would drop to 430km.

Pretty big difference!

last month i drove to QLD... kept her on 95 even when the limit was 100 or 110 (i was in no rush) and got around 610 km!!! (35 litre tank)

cant complain about that

[Walkinshaw]

drag is proporitional to (velocity)^4

[S3SR]

70kms @ 2000rpm will cover more distance and as far as i can tell use no more fuel than doing 60kph @ 2000rpm but will cover 10km less distance per hour than doing 70kph.... i cant remember but are 60kph and 70kph both @ 2000 rpm? and if so change the gear and use less fuel, less rpms - my record of 743kms to 1 tank (motorway/city/suburb driving) by doing that sorta driving..never again though

[monkey_boy1990]

my mums el does better fuel economy at 140kph than it does at 90kph

[monkey_boy1990]

Originally Posted by Yellow_Festiva

Mythbusters have already did this, they used a Lotus super 7 replica (clubman i think with the toyota 1.6 20v engine)

Now they thrashed it, then drove reasonably paced then drove like a granny. Granny mode was the winner over fuel consumption.

I have tested this extensivly with the Festiva, and the way to get maximum km/l is to drive the car in the highest gear possible at the slowest possible speed before you start to labour the engine. So that means i get best results driving in 5th at 55-60km/h

I bought the festiva new, and when running in didnt get over 80km/h. I was AMAZED at how good on juice she was, even bettering the factory claims at times.

Ran her in, started to drive like normal and consumption increased by 20%.

And also driving to uni, mostly express way driving, the faster you go the more juice you use, however i tested the fact that if i went at 80km/h i would get say 550km to a tank, then if i went 100km/h i would get 500km to the tank, but if i kept going at 110-115km/h the range would drop to 430km.

Pretty big difference!

last month i drove to QLD... kept her on 95 even when the limit was 100 or 110 (i was in no rush) and got around 610 km!!! (35 litre tank)

cant complain about that

by the way 5th gear is an over drive so it causes the engine to work harder so be carfull as the gear box may also wear quiker.

[chevypower]

i am sure you will find it pretty much the same litres per 100 doing 60 or 70 in a Falcadore - just use the trip computer

[troppo]

The one @ 70 kph will use more fuel at the same revs because it takes more energy to keep the same mass moving at a higher speed due to increased resistance (wind). Even though it's doing the same revs and speed, it's working harder to do it.

[4.9 EF Futura]

Originally Posted by james22

I would suspect 70 kph, even though the engine is under the same load (2000 rpm),

Load is a 2 variable function... RPM and throttle position.

Im guessing this is an automatic vehicle if we can have 2 different speeds in the same gear at the same RPM...

Throttle position would be the key IMO. Theory would suggest that more throttle required to maintain the higher speed, in which case i'd go for (a).

But a more likely scenario is coasting in the auto, to achieve the different speeds at the same RPM... in which case id go for (b)

[070mstg]

Originally Posted by Yellow_Festiva

I bought the festiva new, and when running in didnt get over 80km/h. I was AMAZED at how good on juice she was, even bettering the factory claims at times.

sorry O/T, but should you buy another new car i think you'll find this method of 'running in' a new engine is quite outdated, anyone correct me if wrong but i think its generally accepted not to 'baby' the engine and to vary the engine load as much as possible

[Interceptor]

best fuel consumption i ever got from my XF was ~250km at 130-140km/h...... only used 31 litres

[05MkIIFutura]

There is absolutely NO CHANCE that 60km/h is the most efficient speed to drive ANY car

As a general rule, 85-95km/h is the most efficient. HOWEVER this is only a general rule and depends alot on the aerodynamics, gearing and power of a car.

I know for a fact that large engined cars which use torque (such as Falcons) rather than power (such as 4 cyl cars) can maintain efficiency up to about 120km/h. The Falcon can still achieve about 8.5l/100km at speeds of 130km/h

There is alot of stuff on this topic scattered all over the internet

Aerodynamic drag depends on the size and shape of the vehicle, its speed (relative to the air), and the density of air.

For a given vehicle, aerodynamic drag is proportional to the square of the velocity.

We define CD as the “drag coefficient”, such that:
Fdrag = 1/2 ..CD.A.v^2
where  is the density of air (1.2 kg/m3)
and A is the frontal area of the vehicle.

[OLDFORDNUT]

ford v8's usually get best economy at 115kmh on Hiway,i think its got something to do with load on drivetrain more than anything else,its there sweet spot ,so to speak

[merlin]

I know one of the Sydney uni's did a study on this a few years back - they concluded that 80km/h was the most efficient speed to drive at.

[ltd]

70K's at 2000 RPm is the most efficient. The coefficient of drag is not proportional to the fuel consumption at that speed, as fords generally have a drag coefficient of 0.34.
Essentially, this means that the car will use the same fuel at 60kmh as at 85kmh per distance travelled.
Unless of course this is a trick question based on the premise that the 4 speed available in the BA does not shift into 4th gear until the car reaches 65-68kmh, which means that the car is labouring at 60kmh.
AU cars changed gears much earlier than the BA's do.
BTW, I set my BA LTD on 70kmh and was chewing 10.4 Litres per 100kmh, set it on 60kmh and was chewing 11.9 litres per 100kmh. The gauge may be off but it does measure fuel flow and is still indicative of the trend. MYTH BUSTED.

[micksta]

so faster at the same revs are better ? (as long as ur going a resonable speed)

[Ives]

Well i've been doing an experiment. My car hasnt seen 3500rpm so far this week and im not getting better fuel economy. I reckon giving the accelerator a bit of a kick wouldnt hurt the fuel economy.... well maybe not if u're not shifting at 6000rpm on every gear

[bathurst77]

IF the car can go 60km/h@ 2000 rpm then it would need over 2000 rpm to do 70. Unles it was going downhill.

[Piotr]

Originally Posted by bathurst77

IF the car can go 60km/h@ 2000 rpm then it would need over 2000 rpm to do 70. Unles it was going downhill.

Downhill or uphill it doesn't matter the gearing won't change because your going down a hill :

The question was hypothetical but it can be done with a CVT.