Assume i am traveling in 5th gear going 110km/h on a highway, holding that speed stready at 110km/h with the accelerator depressed 50% to maintain that speed constant.
Assume then the road became downhill and i dropped it two gears (now 3rd) to slow it down (still a constant 110km/h however due to downhill). Finally after I drop it into 3rd gear, I lift the foot completely off the accelerator as its not needed to sustain the cars speed.

Will I be using more or less petrol when I drop it into 3rd gear and hold 110km/h on the downhill?

I was just slightly confused. Is fuel consumption a component of gear and gas depression, or just determined by how much you depress the accelerator alone?

Thanks guys, feeling a bit noob at the moment. :D

Assume i am traveling in 5th gear going 110km/h on a highway, holding that speed stready at 110km/h with the accelerator depressed 50% to maintain that speed constant.
Assume then the road became downhill and i dropped it two gears (now 3rd) to slow it down (still a constant 110km/h however due to downhill). Finally after I drop it into 3rd gear, I lift the foot completely off the accelerator as its not needed to sustain the cars speed.

Will I be using more or less petrol when I drop it into 3rd gear and hold 110km/h on the downhill?

I was just slightly confused. Is fuel consumption a component of gear and gas depression, or just determined by how much you depress the accelerator alone?

Thanks guys, feeling a bit noob at the moment. :D

And road condition and tyre pressure and.................

Yer diregard those factors. They dont need to be taken into acount. Should have mentioned that earlier.

50% throttle is about 85% airflow compared to 100% throttle

the more you press the pedal, the more fuel you use, so if your not pressing it, its using very very little

the more you press the pedal, the more fuel you use, so if your not pressing it, its using very very little

:stoopid:

Huh? you mean you been using more fuel all this while? Thought it was the basics, luckly you didnt go gear one at those speeds. lol

well i have noticed, that when travelling down steep(ish) hills, i can chuck the car into tiptronic mode and shift back 1 or 2 gears to keep at the speed limit without using the breaks.

EDIT***** the accellerator doesnt get used when travelling down hill, the car is travelling fast enough with the foot off it, and being in a lower gear holds it at the speed limit, so no breaks or accellerator is being used, and it appears that no petrol is either..

At the same time i can have the trip computer set to "instentaneous fuel consumption" and it says it is using no petrol at all and contributes to reducing the "average fuel consumption"

so therefore i'd say it uses less fuel travelling down a hill in a lower gear, and if the trip computer is anything to go by, id say it uses no fuel at all..

would this be good for your engine? i would think not..

From what I thought, the car would have to be using petrol. The more pressure applied to the accelerator, the more fuel it's going to use. If your foot is completely off the accelerator, it'll use very little fuel.

That's what I thought, but the car is revving a fair bit higher in the lower gears at the same speed. I'm unsure so don't listen to me.

it is correct to say that you lower your gear in a down hill situation you do not use more or use less fuel, as it is staying at that speed. This can be felt for those who uses Cruise control function. For arguement sake if you didnt lower your gear, you be speeding in some ways. so its all how you control your car's speed. Some wont mind using the brakes, for your case you use the gears.

it is correct to say that you lower your gear in a down hill situation you do not use more or use less fuel, as it is staying at that speed. This can be felt for those who uses Cruise control function. For arguement sake if you didnt lower your gear, you be speeding in some ways. so its all how you control your car's speed. Some wont mind using the brakes, for your case you use the gears.

no no, i dont use the gear very often, i nearly always use the breaks.. but yeah..

right, situation as follows:

a) travelling along at 60km/ph.. come to a steep decline with an intersection at the bottom, stay in 5th gear (automatic), take your foot off the accellerator and use the breaks to slow you down while travelling down the hill. with the trip computer on "instentaneous" it says around 3.9L/100km and the gears will automatically shift down through 5th>4th 4th>3rd and then normally 3rd>1st as you pull up to a stop.. this way the computer tells me that i am using fuel while going down the hill

b) travelling along at 60km/ph.. come to a steep decline with an intersection at the bottom, stay in 5th gear and move selector into tiptronic mode. Take foot of accellerator with you start to go down hill and in stead of using your breaks to maintain the speed limit while decending, you shift back to 4th then to 3rd, thus making your engine rev higher, but keep your car at about 60km/ph without using the breaks. When i get closer to the bottom of the hill, ill start to use the breaks (instead of riding them down the hill) and maybe shift back to 2nd (at this stage im doing <40km/ph). while doing this, the computer says the "instentaneous" fuel consumption is 0.0L/100km.. this way the computer tells me that im using no fuel while going downt he hill

thats the best i can explain it, im all confused (been up since 4am) but, using cenario A, you rely on your breaks and use more fuel, where-as cenario B you ont use your breaks and use no fuel, but put more strain on your engine (which to me, isnt really the best idea)

with the cruise control activated, it will not lower the gears in automatics to reduce speed while travelling down a hill, you have to slow the car down manually

no no, i dont use the gear very often, i nearly always use the breaks.. but yeah..

right, situation as follows:

snip snip


Just braking I assume the engine is being kept at idle, so some fuel is being added to keep it ticking over.

Using the engine as a braking force, I assume the ECU senses it needs to send next to no fuel to the engine to keep it ticking over as its doing some high RPM with no throttle application.

Just guessing....

...............cenario B you ont use your breaks and use no fuel, but put more strain on your engine (which to me, isnt really the best idea)..............


i dont reckon it can hurt your engine that much, provided you dont throw it back to second at 90k's and havie it slow you down at 6000rpm.... if you keep it <3500rpm it should be fine :)

whenever you are in gear going down hill techincally no fuel is needed to maintain revs but what the ecu actually does is a different matter. gearing down, down hills, is the right thing to do though. more importantly then saving the brakes it prevents them from ever fading on long downhills. as long as you can shift down without slipping the clutch you should do it all the time.

IF you are doing 110km in 5th gear at 3000 rpm on highway going flat, then going down hill in 3rd at 110km, i dare say you would not be using the same amount of fuel, even though you do not depress the accelarater your engine will still be revving much higher than it would be in 5th e.g 5th gear @110kph = "3000" rpm 3rd gear @ 110 kph down hill = 4700 rpm
simple maths people

IF you are doing 110km in 5th gear at 3000 rpm on highway going flat, then going down hill in 3rd at 110km, i dare say you would not be using the same amount of fuel, even though you do not depress the accelarater your engine will still be revving much higher than it would be in 5th e.g 5th gear @110kph = "3000" rpm 3rd gear @ 110 kph down hill = 4700 rpm
simple maths people

no, you wouldnt be using the same amount of fuel.. you would be using less (none even) but your engine is revving higher.. weird..

You pull back a gear or two on the downhill and let engine compression hold your speed - correct. Is your foot on the throttle? If it is then you're using the same amount or petrol the car would need to use to maintain that speed in the gear you've moved down to (lots!). If you're foot is OFF the throttle, no additional air is entering the engine, the TPS is reading NO LOAD, so no extra fuel apart from that used at idle is fed into the engine. The momentum of the car is enuf to keep the engine revving as the car coasts down the hill. Therefore there is no advantage (except mechanical wear) in changing to neutral vs throttle-off compression braking.

would any miniscule fuel savings be worth going out of your way to drive this way though? petrol may be $1.20/l but its still cheaper than milk and i dont know if people use a teaspoon less milk on their cereal to save a few sence...

I don't drink 20L of milk every few days :) And it wouldn't be plesant for the lactose intolerant. It all helps.

Saving $20 of fuel in a year by driving a little differently is worth it. The way I look at it, if you would be pissed off if you find that amount of money missing out of your wallet, then it ranks as a worthwhile measure.

yeh damn fuel these days!Me being the cheap ass that i am have checked a lot of speeds to see the fuel consumption and have found that at 80kms i use above 10L every 100ks but when traveling at 120kms i use 5.6L almost half of what it uses doing 80!!!Also when doing between 0-5kms it uses 14.7L!!!Damn all those traffic lights in penrith making me use so much fuel!!!

The more load such as higher gears and wind resitance and weight at 3000rpm will be using much more fuel than free reving it to 6000rpm in neutral, its all about the load man!

The more load such as higher gears and wind resitance and weight at 3000rpm will be using much more fuel than free reving it to 6000rpm in neutral, its all about the load man!
And load is measured by the TPS, which if the TB is closed - minimal load - less fuel used (assuming the car is actually moving)