From the previous threads about driving a manual, some people have said that downshifting to stops istead of shifting to neutral and coasting will use more gas (downshifting that is).

Is this true or false and can you provide a web link or a reference for proof?

 

It's false. However, you won't coast as far with the car in gear since you have the drag of the engine. It's called engine braking, and although the engine would be at RPM above idle, it's only getting as much fuel as it would at idle because the throttle is closed.

 

I dont believe that is correct.

If you simply consider the physics... An engine running at higher RPM will burn more gas than an engine running at lower rpm.

The throttle might be closed, but the engine will still be fed the gas it needs for the rpms to go up during a downshift.

 

Wrong.

 

not that I'm questioning either of you, but do you guys have any data (i.e. website) to back any claims?

 

I can't see how an engine spinning at 3000 rpm uses less fuel than the same engine @ 1000 rpm. Every 4 strokes gas is shot into the engine, no? If there's more strokes there's more fuel used. There's no doubt that when you open the throttle it will shoot gas in in larger amounts but that doesn't mean it stop using fuel when it's closed. I'm no expert but just applying logic to what I already know....could be wrong though.

I know someone with a real time fuel economy gauge on thier new truck. Next time I am in it I will keep it in 1st gear and rev it up and see what the real time fuel usage is while the engine is slowing it down compared to being in neutral

 

looks like this one's gonna be going for a while...

 

im no expert on this, in fact, i have an automatic

but i heard somewhere that if u shift to neutral at stops, ur making ur engine do twice the work or something and therefore burns more gas

is that correct?

 

I totally agree. The key here is that the engine at 3000 RPMs slowing down after a downshift (no throttle being applied) and idling to a stop in neutral (no throttle applied) should use the SAME amount of gas. The engine is spinning at 3000 RPM in the first case becasue the momentum of the car is turning the engine that fast, not the fuel from your throttle.

 

I don't know if this is any consolation, but hard braking has been proven to use a lot of gas. The comment about the throttle and the engine still burning fuel, the amount of fuel delivered to the engine is not dependent upon engine rpm, it's dependent upon throttle and the amount of air being drawn into the vehicle's intake system. If you're simply down shifting, you're not gonna be on the gas, therefore, the engine will be drawing in less air and you'll just be burning fuel that you normally wouldn't have burnt (which our catalytic converters disperse into safer emissions). Downshifting will actually save you gas over using the brakes. Letting the engine slow the car down by utilizing increased burning of already delivered fuel will use a lot less gas than applying pressure to the brakes to bring the car to a stop. Generally, when engine rpm is high and air intake is low, the ocmputer compensates by not delivering more fuel than required to match airflow. Hope this helps.

 

I think that the fuel consumption will be negligible either way this arguement ends up.

 

I'm clueless :? I'll just turn it off whenever i'm coming to a stop, that's gonna save me a lot. Maybe

 

its true, everytime your RPM goes up in numbers, the cars engine supplies more fuel to the cylinders.

just like hard accelaration and redlining the car 24/7 will definetly waste more gas!

plus downshifting also takes a toll on your clutch...just my 2 pennies, been driving around my girls 5spd black cherry and i love the shifter so im also downshifting it alot

but yeah downshifting will eat up more gas

very little difference but there is a difference(esp. if your a penny pincher...hehe...JK)

 

This arguments does not seem to be ending. I dont know what the answer is either, but maybe I can help out. Here are the two possible answers and the logic behind them (in my mind atleast):


Engine breaking uses more gas
Since the RPMS are higher the engine is drawing in/ forcing in more gas for each revolution.

Engine breaking uses less/ the same gas
The engine is useing the same gas as idle and it is the turning wheels that keep the engine going, not additional gas.

I am belive that both of those are true, I think it is the wheels rolling that keep the RPMs up, but since the RPMs are up a little more gas is probably being sucked into the engine, but not forced in. I think to know the answer we want someone that knows modern engines well. It is very possible the CPU could be micro-managing the gas during engine breaking. It could be cutting all gas since the wheels will keep the engine turning. Or the CPU could opening up the throttle to give it the gas for that RPM.
Either way, I dont think engine breaking is going to make that big a difference in your milage.

 

These are my thoughts as well... I cant see it being possible that an enging running at 3,000 rpms burns the same amount as 700 rpm idle speed (i.e. downshifting vs. neutral coasting.

Even if the throttle is not feeding extra fuel, the cylinders still have to fire the extra amount of times. I was under the impression that cyclinders firing required fuel.

 

That's my point though, your engine isn't RUNNING at 3000 RPM's, it's being forced to turn that fast by the weight of your car.

 

The fact of the matter is that a fuel injected engine doesn't "suck" fuel at all. It's injected in the amount needed. When the engine is downshifted, the ECM goes into fuel cut. NO FUEL. It takes fuel to idle an engine but no fuel to motor the engine during deceleration. I think gentle driving, however you like will save the most fuel. I personally am too lazy to downshift that much, but I don't drive all that fast to begin with.

 

ok, but if the engine is turning then it will be firing.

it cant be turning over and not firing.

 

I really hope this helps.

It will use more gas, but only a minute amount. It does not get the same amount of fuel it would with the throttle open at 3000, it will get less, that's why the vehicle slows down.

 

This is from an "ask the mechanic website" :

Q:Does it matter if you stop by downshifting or by braking? I'd heard downshifting is hard on the clutch while braking is hard on the brakes. Are either of these true?

A:Now as for downshifting, it is cheaper to change brake pads than it is to change a clutch!

**************************8

Downshifting can be hard on a clutch, especially the pilot bearing. I would rather replace some brake pads than take a hit on having to replace a clutch and pilot bearing prematurely. What's the hit on a clutch replacement on the tC? I know it cost me OVER $700.00 to replace the clutch on my Honda Civic.