Scenario 1: 7000rpm, 1st gear, 30mph
Scenario 2: 7000rpm, 5th gear, 130mph

If you started both scenarios with a full tank of fuel in which would you run out of fuel quicker (in terms of time, not distance)? Or would it take the same amount of time to run out?

edit: Full throttle in both cases, the car has a soft rev limiter at 7000rpm.

air resistance a factor no?

At higher speeds there is more resistance, so more fuel usage.
In 1st gear you don't have to push throttle all the way down to get 7000rpm.

I'd think it wouldn't make the difference, since if both cars are starting their journeys at that that speed (how? Reverse Italian Job style?), they didn't need to overcome the air resistance in order to get there. So 7000rpm in neutral would probably do the same, although the engine would probably explode. Maybe air intake and temperature need to be taken into account in these situations. The car at 130mph will surely be cooler than the one at 30.

I'm no scientist, I'm a media student. I know nothing!

Quote from G!NhO :

In 1st gear you don't have to push throttle all the way down to get 7000rpm.

You're right but assuming that he means it's being run against the rev limiter I'm gonna answer 'nearly the same' (if the limiter is done by cutting the injection) and '~exactly the same' if the spark is cut

usage per distance should be higher with the 1st gear example as the car isn't covering much distance very fast yet the engine is near or at maximum consumption.

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Maybe air intake and temperature need to be taken into account in these situations. The car at 130mph will surely be cooler than the one at 30.

valid point

My driving instructor said that lower rpm's mean more fuel usage. True? Just a "fact" that i just reminded.

So that would be like, on low rpms, engine goes hotter than on high rpms and fuel usage is bigger too. Lets all drive on max rpms!

Quote from hazaky :

My driving instructor said that lower rpm's mean more fuel usage. True? Just a "fact" that i just reminded.

So that would be like, on low rpms, engine goes hotter than on high rpms and fuel usage is bigger too. Lets all drive on max rpms!

Crazy instructor.

At 7000rpm, you would do more miles in 5th rather than in 1st (edit: using 1 galon of fuel). You can count in the air resistance and engine temperature all you want, but in my opinion it all comes down to gear ratios.

...and doing 7000rpm for a longer amount of time surely isn't healthy for the engine anyway.

Quote from hazaky :

My driving instructor said that lower rpm's mean more fuel usage. True? Just a "fact" that i just reminded.

So that would be like, on low rpms, engine goes hotter than on high rpms and fuel usage is bigger too. Lets all drive on max rpms!

I'd recommend an instructor change ASAP..

Bang Goes the Theory disused this (and I just happen to be watching it now), and the faster you go the more you up your consumption due to be using more power to overcome the drag.

If you could remove the drag from the equation then consumption would be virtually the same as you idle at, naturally not possible, so lower speed would use less fuel.

But is the power not the same as the engine is at full throttle and at max revs?

Quote from J@tko :

But is the power not the same as the engine is at full throttle and at max revs?

It has the same power but at higher speeds it has to overcome more resistance and so it will use more fuel, i guess.

But the question is saying that the cars have started their journey at the speed they are travelling somehow. And 7000 rpm is 7000rpm in whatever gear you're in, which is surely the same amount of fuel. Maybe I'm not getting this.

Either way, faster speed = higher wind resistance until its all balanced out. No?

So. No matter what speed it starts at, the final constant speed will remain the same. 130mph would have stronger wind resistance.

Thats how I understand it.

Quote from G!NhO :

It has the same power but at higher speeds it has to overcome more resistance and so it will use more fuel, i guess.

That's what is kinda confusing me.

An engine produces power by making fuel explode. The amount of fuel used depends on how many explosions per second [i.e. RPM]. The amount of power needed is equal to the forward force x the velocity of the car. As Force = Mass x Acceleration, and the acceleration is 0, the only force needed is to overcome the resistive forces [drag and friction]. So I think that they use the same amount of power [as the RPM is the same], but in the lower gear some of that is wasted as the power needed is lower [resistive forces lower, velocity lower] then in the higher gear, thus the fuel consumption is the same.

I could be totally wrong, however

Quote from J@tko :

That's what is kinda confusing me.

An engine produces power by making fuel explode. The amount of fuel used depends on how many explosions per second [i.e. RPM]. The amount of power needed is equal to the forward force x the velocity of the car. As Force = Mass x Acceleration, and the acceleration is 0, the only force needed is to overcome the resistive forces [drag and friction]. So I think that they use the same amount of power [as the RPM is the same], but in the lower gear some of that is wasted as the power needed is lower [resistive forces lower, velocity lower] then in the higher gear, thus the fuel consumption is the same.

I could be totally wrong, however

That's how I understand it, but I couldn't put it into words as eloquently as you.

But this is without the point I made earlier, of temperature being a factor. But maybe we should forget that little complication

Quote from Mp3 Astra :

That's how I understand it, but I couldn't put it into words as eloquently as you.

Who knew an internet forum would be such good revision for my mechanics exam?

Quote from DevilDare :

Either way, faster speed = higher wind resistance until its all balanced out. No?

So. No matter what speed it starts at, the final constant speed will remain the same. 130mph would have stronger wind resistance.

Thats how I understand it.

You understand it correctly, and the resistances would play a role if we were talking about distance of travel, or speed of travel. But does it play a difference with the engine at the same RPM. Essentially the engine is doing the same thing, the transmission is doing different things. The transmission does not use fuel, just takes the engine output and converts.

Now, the kicker comes into play with engine load. You can rev to 7000 and keep it there in first a lot easier than in 5th. Because of resistances. So does this extra load on the engine require more fuel, I would say most likely. Least it makes sense to me.

Be just like a peddle bike, in lowest gear you can only peddle so fast before your feet fall off. For giggles lets call that 7000rpm. Then you change the gears to the highest and do the same thing. But as soon as you slow down your legs have a load on them and you need push harder again. This is where more fuel/energy would be used. But assuming a magical world where you could actually keep your legs at that speed, (or the car engine), then it might be using the same fuel.

My bet is that 5th gear will run out of fuel sooner (time-wise) because it will have more load on the engine running at 7k where as even if the engine is running at 7k in first, it won't have the same load.

I guess Scenario 2 should run out of fuel much quicker. Because of the drag it takes more energy to maintain a speed of 130mph than a speed of 30mph. So the fuel consumtion should be higher.

Even if you do 7000 rpm in both situations, the amount of fuel injected is computer-controlled depending on the load, thus it can be completely different.

Quote from Matrixi :

I'd recommend an instructor change ASAP..

No need for that, hes a nice guy infact. Maybe he meant that if im on low rpms and then hit the throttle. It makes sense a bit to me, but im not sure if im right. Im goin to drive in 7 hours tho.

Quote from blackbird04217 :

My bet is that 5th gear will run out of fuel sooner (time-wise) because it will have more load on the engine running at 7k where as even if the engine is running at 7k in first, it won't have the same load.

1st up a very steep hill and 5th on a level road...

Assuming the ECU knows throttle opening and RPM only, and injects fuel and sparks sparks as defined for those two variables, then both scenarios use the same fuel if these two things are the same. Thus steep hill in 1st is the same as flat straight road at Vmax in 5th.

And no, it's not as simple as "you use more fuel at lower RPM". Each injection pulse might be bigger (biggest at maximum torque in theory), but it's offset by less injections per second. You'll use the most fuel when doing the most work - accelerating hard, ploughing, climbing a steep hill or cruising at high speed. Tootling around like a granny and asking very little of the engine generally uses less fuel. But tooling like a granny and taking a whole minute to reach 50mph MIGHT use more fuel than accelerating like a chav to 50mph in, ohhhh, 14 seconds (they have shit cars and heavy bodykits, so their cars aren't quick) then cruising for a bit.......

Hmm, I always thought the more air the car gets the better it will perform and use more fuel. Thus when driving faster(higher rpm) the car needs more air.

But since were talking about the same RPM's, I'm really clueless. I WANT ANSWERS!

I guess this question is a bit more complex and I'll demonstrate why Let's try to figure out which scenario will consume fuel quickly time-wise.

Say you have a dyno with variable resistance. First you set it to create a resistance equal to aero drag you'd get at 30 mph. You rev the engine up to 7000 rpm at 1st gear and measure the time needed to empty a full tank. Result would be, say, 2 hours. Now you do the same, but you set the resistance to 130 mph aero drag and run on 5th at 7000 rpm. You'll find that the tank was used up sooner, say, in 1 hour.
Why? Revving the engine up to 7000 rpm at 1st doesn't require you to push the pedal to the metal. Less throttle, less air in the combustion chamber. The less air there is, the less fuel is needed. Less fuel, lower consumption. On the other hand, revving the engine up at 5th needs full throttle. I guess you can figure it out from here

Now let's do the same test on an actual road. If you travel at 30 mph for 2 hrs, you travel 60 miles distance. Traveling and 130 mph for 1 hr gives you 130 miles distance. So in this case, your MPG would be more that doubled when running flat out on the 5th gear, even though the amount of fuel per second would be higher... pheww, hope you get it...

How is it so hard to understand the fact that if in both scenarios you start out at that speed with the same amount of fuel, in scenario 2, you will need to have the car push harder through the air, (it has more wind resistance) there for will consume a bit more fuel to maintain that speed at that RPM?

The only possible way they could use the same amount of fuel for both scenarios is if they were on a dyno of some sort, or you change a lot of other factors like air and engine temp., air density, humidity, fuel temp, oil temp, and other things like that IMO.

Quote from tristancliffe :

1st up a very steep hill and 5th on a level road...

Your correct but I was assuming a proper scientific method where we eliminate as many variables as possible - the only thing changing is the gear, and thus the speed. Try doing 5th up that same steep hill.

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And no, it's not as simple as "you use more fuel at lower RPM". Each injection pulse might be bigger (biggest at maximum torque in theory), but it's offset by less injections per second. You'll use the most fuel when doing the most work - accelerating hard, ploughing, climbing a steep hill or cruising at high speed.

Exactly what I was saying above with the engine at load or not. The resistances in 5th will be higher than those in first. Causing the engine to do more work.

If the engine always took in X fuel per rotation, and produced Y torque/energy from that fuel, then your speed in 5th would be limited to where the resistances match the energy output - regardless of engine speed... So at that point the engines would take the same amount of time until they ran out of fuel, but the one in 5th would not be running at 7000 rpm, leaving the question unanswered.