Fuel Consumption - 60kph Vs 70kph

[shane3]

Originally Posted by monkey_boy1990

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.

Where the **** did you learn about gear boxes!!! "over drive" is a taller gear so the car will do a greater speed at lower revs, the O.D in most cars is for high speed cruising (long distances)

Take you BA XR6/8, FPV GT 4 speed auto, 3rd gear should be a 1:1 ratio and 4th gear being the over drive will be about 0.65:1, basically you get more out then you put in!

[4.9 EF Futura]

Originally Posted by Piotr

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

With an automatic transmission (or more to the point, with a viscous coupling instead of a clutch/flywheel) there is scope for a non linear relationship between RPM and speed. No the gearing doesnt "change", but slip can occur within the torque converter...

Or yes... CVT....

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

RPM is irrelevant without putting it in the context of load. i.e. you'll use a lot more fuel lugging something up a hill in top gear at 1500rpm than you would by choosing the correct gear and utilising a more favourable part of the engine's powerband (and also providing a significant torque advantage by selecting the right gear).

Similarly, you can have it in first gear and take forever and a day to creep up to the redline by resting a featherweight on the throttle... its not going to use much fuel because the engine is hardly under any load. Sure, with those RPMS the injectors are firing "more" frequently... but they'll be injecting SFA each time....

[jzab]

Originally Posted by HyperKid

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.

This is a darn good question and it so depends on the car you're talking about and the engine its fitted with. I thought I'd add a bit of science to this so I did some calculations all based around the BF Falcon XT 4speed auto and came up with the following. Please note all the data here is taken from testing and anything I thought might be confidential I have not included in my post. It takes about 5.8kW at the wheels to drive the XT at 60km/h and 7.8kW at the wheels to drive it at 70km/h. This increase is due to higher wind resistance, rolling resistance and friction in the drivetrain. The numbers also assume a run-in and fully warmed vehicle (including engine, transmission and diff), fully pumped up tyres that have also been 'run-in' and a perfectly flat bitumen surface and no head or tail wind. If you assume that the drivetrain losses are the same for both (and is 30%), then at 60 km/hr the fuel consumption would be 4.37l/100km and at 60 km/h it would be 4.44l/100km or 2% more fuel consumption at 70 km/hr. The reason they are closer than you would first think based on the road loads is that the BF engine is more fuel efficient (about 10% more efficient when measure in grams/kWhr) producing the higher power required for 70km/h but this is entirely cancelled out (+2%) by the higher drag.

Now of course you would never match those fuel consumption numbers on the road, because you can not drive under those ideal conditions all the time, but they are within the scope of the question. Also if you raised or lowered the speed, and added hills and corners in to the equation, you might end up with a different answer.

I hope this helps, or perhaps it is just a catalyst for further discussion!

Jzab

[xbgs351]

Given that in both scenarios the cars are doing the same revs, I would expect the car to use more fuel at the higher speed as there is more wind resistance.

That being said, I have driven cars that used 25% more fuel at 80 km/h than 120 km/h. The answer to your question will depend on the car.

[Conrad]

Originally Posted by D1XR2C

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...

120 if ur lucky.