new manual driver: does this destroy my tranny?

[DrDuquette]

Feathering the clutch will wear the clutch plates.

Don't take it out of gear. Everyone is right. Idleing will used more gas.

Engine braking isn't bad for the engine. Tell that to the trucker who use j-brakes.

To slow down let off the gas and then use the brakes to make your complete stop or to stop fast.

You should rev match if you are making big downshifts though.

[luxmn]

Originally Posted by 7thgear

if done improperly it will, severe shock to all components if you clutch dump into 2nd while doing 100kph or something.


ever see a car's RPM needle go past the tachometer faceplate? ask me how i know.

i've done this a few times already, by accident of course, from 6th (going 70mph) to 3rd (instead of 5th) and the clutch barely lets me do it. I think there's a safety mechanism where if your speed is too high the clutch won't let you dump into 1st unless you use a lot of your body weight to avoid accidental downshift.

not that i would ever try it.

dumping clutch to 2nd at 70mph is not braking, is called being retarded.

[serialk11r]

Okay you are doing what's called engine on pulse and glide. Your top speed is probably too high to be saving any gas, try to keep the maximum and minimum speed close together.

It saves gas only because engine efficiency is high when you step on it and accelerate, and idle consumption is probably like 10 times lower than cruise consumption so it is almost negligible (hardcore hypermilers turn the engine off and then bump start it to maximize savings). Under constant speed cruise at whatever average speed the engine is only at perhaps 70% peak efficiency, while when accelerating you can be 90-100% of peak efficiency.

Lifting your foot does NOT save gas if you do not want to brake. What it does is use up the car's kinetic energy to spin the engine. While this sounds like it's no different from idling, the transmission transmits power less efficienctly backwards, and when the fuel is cut the throttle is also closed, so you have very high pumping loss because you lose energy on both the intake stroke and the expansion stroke, whereas under idle the expansion stroke loss is smaller. I believe if you ever lift your foot completely you'll be braking, so try to avoid lifting your foot all the way unless you're going down a hill and don't want to keep speeding up.

If you really want to save gas, cut the 85mph down to like 70, but you'll need to use the clutch more so keep in mind that you're putting a slightly greater amount of wear on it. Or, just leave it in 6th, drive 60-65, and take the ~10% gas penalty compared to 70-55 pulse/glide, and don't break any possible laws against coasting in neutral

[EsoBOFH]

Originally Posted by Saibot

I've recently just learned how to drive manual. To save gas and engine wear; recently I've been coasting on the highway at high speeds.


Ie, I'd accelerate to 135kmph (85mph) in 6th gear then clutch in, go to neutral and coast down to 90 kmph (55mph). I would then blip the throttle, and shift back into 6th and repeat.

Someone told me this increases wear on the tranny; but I'm not sure if this guy knows what he's talking about.

Can someone shed some light? Thx.

Pointless, and subjects your tranny/clutch to more cycles then necessary. It takes more fuel to accelerate your car (especially when your talking the speeds you are talking, battling wind resistance, etc) than it does to just simply maintain a steady, reasonable speed.

Travel at the speed limit, in highest gear to save wear and fuel. Use the cruise control.

[WolfsFang]

Originally Posted by EsoBOFH

Pointless, and subjects your tranny/clutch to more cycles then necessary. It takes more fuel to accelerate your car (especially when your talking the speeds you are talking, battling wind resistance, etc) than it does to just simply maintain a steady, reasonable speed.

Travel at the speed limit, in highest gear to save wear and fuel. Use the cruise control.

this is true, i set my cruise at 70mph and i was getting 32mpg on flat roads.

[Thunderchicken]

Just my 2 cents never coast in neutral it's very dangerous for you and the people around you for many reasons 1: allows you to acclerate out of danger i.e. critter in road you can acclerate minor correction to avoid running something over about 70% of most accidents can be avoided with more of the skinny pedal. 2. When you are in gear you are able to drop a gear and brake to stop much quicker than brakes alone.

[7thgear]

Originally Posted by serialk11r

Lifting your foot does NOT save gas if you do not want to brake. What it does is use up the car's kinetic energy to spin the engine. While this sounds like it's no different from idling, the transmission transmits power less efficienctly backwards, and when the fuel is cut the throttle is also closed, so you have very high pumping loss because you lose energy on both the intake stroke and the expansion stroke, whereas under idle the expansion stroke loss is smaller. I believe if you ever lift your foot completely you'll be braking, so try to avoid lifting your foot all the way unless you're going down a hill and don't want to keep speeding up.

momentum is free energy as far as the car is concerned

i don't have the math, but my gut feeling tells me that accelerating gently from lets say 100kph to 105kph in 6th gear will require less fuel than the distance the car traveled while it decelerated from 105 to 100.

if you can prove me wrong, i'll only be glad. (also i realize that this is highly influenced by the rolling resistance of the tires and drag of the vehicle)

also traffic is a fucked up variable. Many people brake on the freeway when they don't have to, and then compensate by applying more throttle to get back up to speed

so in those long drives on the freeway at speeds of 90-120kph, lifting to coast instead of neutral or using brakes is the best way to save gas.

[Slartibartfast]

Always keep the car in gear. You never know when you need power. As for rev-matching on downshifts, I can tell you that 90% of people that drive manual don't, but 100% should. When I was a salesperson years ago, I was amazed that, over a 8 year span, with about 4 test drives a day, I had one (one!) test drive where the customer matched revs.
It's not hard, but takes a bit of practice and finess. Practice practice practice. When you get good at it, it makes driving stick more fun and involving.
And, when you are comfortable with that, you can start doing heel/toe. Even more fun!

[Sportsguy83]

Originally Posted by Chewie4299

Coasting in neutral uses gas to keep the engine at idle. Coasting in gear uses no fuel. The wheels are effectively turning the crank so the ecu cuts fuel.

Originally Posted by 7thgear

if you just lift the gas pedal then the car will coast and consume zero gas until your rpms drop below 1200 or so.

Originally Posted by DrDuquette

Everyone is right. Idleing will used more gas.

Wow... I've been into cars for over 10 years and did not know that. I read online after looking at the thread and it seems all of you are right. I always thought even at no throttle, the ECU would always feed the engine the required fuel for certain RPM's (keeping the throttle open to get air as needed accordingly). I will stop cruising in neutral (which I do a lot because I was sure I was saving gas LMAO). I will keep an eye on MPG per fillup.

[the_3d_man]

No one has mentioned that when accelerating the ECU switches to open loop mode, vs. closed loop for constant load cruising. This is the main reason why accelerating and coasting is less fuel efficient. And it's stupid as other pointed out because you never know when you will suddenly need a little power.

And to engine braking, not necessary unless going down a long/steep hill. For normal braking, unless you rev match perfectly every time, you are increasing clutch wear. Brake pads are cheap and easy. For panic breaking you don't have time to be messing with a downshift, and the shock of dumping the clutch while braking could cause you to break free. The brakes provide more than enough stopping power and I don't think anyone can prove that engine braking allows you to stop faster.

[ThatGuy]

Originally Posted by sportsguy83

Wow... I've been into cars for over 10 years and did not know that. I read online after looking at the thread and it seems all of you are right. I always thought even at no throttle, the ECU would always feed the engine the required fuel for certain RPM's (keeping the throttle open to get air as needed accordingly). I will stop cruising in neutral (which I do a lot because I was sure I was saving gas LMAO). I will keep an eye on MPG per fillup.

Yes, newer cars to do not use any fuel on deceleration in gear.

Back to the OP Yes, this would cause some increased wear on many gearbox and clutch components:
1. clutch friction surface
2. flywheel surface
3. throw out bearing
4. slave cylinder seal
5. master cylinder seal
6. clutch pedal lining, spring, return bumper.
7. shift linkage bushes


While there might be VERY small gains to be made by putting the clutch in an idling for a few seconds then rev matching and slightly accelerating I think in practise you would find it almost impossible to achieve any significant savings.

Thermodynamics tells you the most efficient method is to be as close to a steady state as possible. The change in speeds is definitely inefficient. ie. going 75mph, 65mph, 75mph, 65mph is definitely going to hurt economy.

The best way to save fuel is to:
Be in the highest gear that does not cause engine lugging.
Be gentle with throttle inputs
Look ahead and adjust speed before you have to (try to max speed transition as slow as possible, or remove them altogether) **** This is the BIGGEST factor in real world fuel economy.****

Saving fuel during cruise will make little difference to total fuel consumption.

[Razz]

Originally Posted by Saibot

I've recently just learned how to drive manual. To save gas and engine wear; recently I've been coasting on the highway at high speeds.


Ie, I'd accelerate to 135kmph (85mph) in 6th gear then clutch in, go to neutral and coast down to 90 kmph (55mph). I would then blip the throttle, and shift back into 6th and repeat.

Someone told me this increases wear on the tranny; but I'm not sure if this guy knows what he's talking about.

Can someone shed some light? Thx.

You got to be kidding.

[serialk11r]

Okay I'm not going to try to address specific posts, but I will attempt to explain "pulse and glide" to people, as well as fuel cutoff and idle.

If your drivetrain had near constant efficiency, then traveling at a constant speed given some average speed is certainly most efficient, because energy consumption goes up the faster you go. The big if is the drivetrain being constant efficiency. At idle your engine is near 0% efficient. On the road, depending on gearing and such, it is typically consuming 50% more fuel than ideal, that is the efficiency is less than 70% of peak. The rare moment when your engine is operating close to peak efficiency is when you're stepping on it. Let me explain this further...

Nearly all hypermilers use pulse and glide, even if some don't quite understand how it works and speed up too much. This allows them to get much higher highway mpg than you'd ever think is possible. If you don't think it's effective, poorly geared cars can get 10-20% better highway mpg through this technique. The reason is because your cars are geared so that in highest gear, it still has reserve power for hills and wind and such. For example on the FRS the load on the engine is about 25% at 60mph.

By contrast, typical car engines are most efficient at about 70% load. In short, stuff like friction being mostly rpm dependent rather than load dependent, along with throttle losses mean that an engine is more efficient at higher load, up till the point where efficiency is being sacrificed to protect the engine and make a little bit extra torque: namely fuel enrichment past stoichiometric. Now 25, 70 are very far apart, but it's not that grim: typically 40-80% load is all pretty efficient, in some sense. When you floor the gas and rev all the way to the limiter, the efficiency is similar to highway cruising.

What pulse and glide does is vastly increase the efficiency of your engine when it's providing power, and this is typically enough to overcome the extra energy you spend going faster momentarily and then slower, so much that even if you let the engine idle it will give an increase in efficiency. Now the wider your speed interval, the less effective this is, and you'll hit a break even point. 85mph as the OP states is quite excessive.

You might wonder why it's going to use less fuel to idle than to just lift off the gas. When you lift off the gas, the throttle closes, and the engine's specific frictional+pumping power is at its greatest. Moreover, the energy the engine is using up came from the kinetic energy of your car. Where did that come from? It came from the engine originally! Except now, the energy travels back through the tires and transmission, and experiences another ~15% loss. So engine braking should strictly be for braking, not coasting. If you need to stop the car or slow it down, sure use the engine, no fuel is better than the small amount of fuel used to idle and it saves your brakes too.

Now all that said, I'm not advocating pulse and glide, with the following distances people usually have on the road, you'd end up scary close to people. Too much of a hassle to shift all the time, and will encourage trouble on the road with other drivers.

[Thunderchicken]

Originally Posted by serialk11r

Okay I'm not going to try to address specific posts, but I will attempt to explain "pulse and glide" to people, as well as fuel cutoff and idle.

If your drivetrain had near constant efficiency, then traveling at a constant speed given some average speed is certainly most efficient, because energy consumption goes up the faster you go. The big if is the drivetrain being constant efficiency. At idle your engine is near 0% efficient. On the road, depending on gearing and such, it is typically consuming 50% more fuel than ideal, that is the efficiency is less than 70% of peak. The rare moment when your engine is operating close to peak efficiency is when you're stepping on it. Let me explain this further...

Nearly all hypermilers use pulse and glide, even if some don't quite understand how it works and speed up too much. This allows them to get much higher highway mpg than you'd ever think is possible. If you don't think it's effective, poorly geared cars can get 10-20% better highway mpg through this technique. The reason is because your cars are geared so that in highest gear, it still has reserve power for hills and wind and such. For example on the FRS the load on the engine is about 25% at 60mph.

By contrast, typical car engines are most efficient at about 70% load. In short, stuff like friction being mostly rpm dependent rather than load dependent, along with throttle losses mean that an engine is more efficient at higher load, up till the point where efficiency is being sacrificed to protect the engine and make a little bit extra torque: namely fuel enrichment past stoichiometric. Now 25, 70 are very far apart, but it's not that grim: typically 40-80% load is all pretty efficient, in some sense. When you floor the gas and rev all the way to the limiter, the efficiency is similar to highway cruising.

What pulse and glide does is vastly increase the efficiency of your engine when it's providing power, and this is typically enough to overcome the extra energy you spend going faster momentarily and then slower, so much that even if you let the engine idle it will give an increase in efficiency. Now the wider your speed interval, the less effective this is, and you'll hit a break even point. 85mph as the OP states is quite excessive.

You might wonder why it's going to use less fuel to idle than to just lift off the gas. When you lift off the gas, the throttle closes, and the engine's specific frictional+pumping power is at its greatest. Moreover, the energy the engine is using up came from the kinetic energy of your car. Where did that come from? It came from the engine originally! Except now, the energy travels back through the tires and transmission, and experiences another ~15% loss. So engine braking should strictly be for braking, not coasting. If you need to stop the car or slow it down, sure use the engine, no fuel is better than the small amount of fuel used to idle and it saves your brakes too.

Now all that said, I'm not advocating pulse and glide, with the following distances people usually have on the road, you'd end up scary close to people. Too much of a hassle to shift all the time, and will encourage trouble on the road with other drivers.

When coasting in neutral, the engine is idling, consuming just as much gasoline as when it's idling at a traffic light or warming up in your driveway, roughly gallons per hour (gph), depending on your vehicle. Some small cars with roller cams may do slightly better, but a rule of thumb for idling fuel consumption is 1 gph . Let's use 1 gph as a starting point, just to make the math simpler.

Suppose you are coasting down a mile-long hill at an average speed of 30 mph, which will take close to 2 minutes. During this period, you'll consume approximately 0.033 gallons of gasoline, for a consumption of 30 mpg.

Read more: Coasting in Neutral or Gear to Save Gas - Coasting and Fuel Economy - Popular Mechanics