MrViet

my friend shifts around 3-3.5k and we argue that shifting at a lower rpm at like...2k rpm and going 5th gear on local saves more gas, is this true?

BUST THE MYTH ! shifting low rpm(like 2k) and doing 5th gear on local saves gas?

i think its time we crack down on this myth.

kellogg97rs

i believe FALSE...Shifting too early will cause you to bogg down and you will use more gas getting up to speed. You want to shift at the rpms the owner's manual tells you too for normal driving...usually around 3k (depends on car/engine)

MrViet

can i get another to confirm?

BlueIntegraBoy

iTrader: (5)

What gear are you going into 5th from? Don't tell me from 2nd to 5th.

MrViet

no no no just normally from 1 - 2 - 3 - 4 -5 but shifting at 2k rpm.

dumada9

i believe its right that shifting a too low of an RPM will cause you to waste more gas because you have to push the car more to accelerate.

Dogginator

It depends. You need the HP/(g fuel) vs. RPM plot for the engine. (I posted this a long time ago here for the JDM ITR engine and don't care to search for it.) In general, WOT is the most efficient. For the JDM ITR engine, the most efficient operation was between 3-4k RPM at WOT.

The thing to avoid to maximize efficiency is using the brakes. Every time you use those, you convert mechanical energy to heat.

cdntegintx

Keep in mind, that alot of the time its the throtte position sensor that determines the amount of gas that goes into the engine. Meaning that if your in 4th gear with the TPS at 10%, you'll probably use less gas than in 5th gear with the tps at 20%. So in order to use the least amount of fuel, you want to be in the gear that will allow you to travel at the required speed with the least amount of pedal movement.
Also keep in mind that this is relative. If you are in 1st gear at 8000 rpm vs 3rd gear at 2000, you may use more because the engine is spinning 4 times more. As a rule, try to be in the gear that allows you to have the least amount of throttle input.

posHonda

If youre at too low of an rpm and have to give it gobs of gas to move than yea it will use more gas than if you just revved it higher.

jhammond_82

iTrader: (1)

i agree, the harder the throttle is pressed, the more fuel is dumped in. period. i don't think it would matter if your at 8 grand or 2 grand, the position of the throttle determines how much fuel is used. and even if it did save gas, i'm sure it's not enough to really notice a difference at all. when you have a car that gets 30 mpg, why would you need better, thats pretty damn good.

D@9 A.D.B.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by cdntegintx &raquo;</TD></TR><TR><TD CLASS="quote"> So in order to use the least amount of fuel, you want to be in the gear that will allow you to travel at the required speed with the least amount of pedal movement.
</TD></TR></TABLE>

they both sound about right to me

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by kellogg97rs &raquo;</TD></TR><TR><TD CLASS="quote">You want to shift at the rpms the owner's manual tells you too for normal driving...usually around 3k (depends on car/engine) </TD></TR></TABLE>

Dogginator

TPS controlling the amount of fuel.

I'll be nice. So here it is. The amount of fuel fed into the cylinder is determined by the oxygen sensor during normal operation. The TPS has little to do with the actual amount of fuel fed into the engine: it serves more as a predictictor to changing fuel needs as the throttle position changes.

The exceptions to the oxygen sensor control is open loop (cold engine) and WOT.

When cruising, you depress the throttle to the point where you make enough HP to maintain speed. In general, lower RPM operation requires greater torque to produce the same HP as high RPM operation. To make this greater torque, the cylinders are more fully filled compared to high RPM making the same HP. Fuller cylinders creates a higher dynamic compression ratio, which operates more efficiently than partially filled cylinders.

mac_24_seven

iTrader: (1)

I go between 2-3,000 rpm when I shift.. Depends on my mood.

When I'm in my "Ultimate gas savings mode" I shift around 2,000... But I take a long time to accelerate too, to get to highway speeds.

But honestly, I'm rather particular about my gas mileage and with easy driving, I havn't noticed much difference at all between shifting @ 2,000 RPM or shifting @ 3,000 RPM.

Now, going 55 mph on roads compared to 65 mph: Thats another story.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Dogginator &raquo;</TD></TR><TR><TD CLASS="quote">The thing to avoid to maximize efficiency is using the brakes. Every time you use those, you convert mechanical energy to heat.</TD></TR></TABLE>

He speaks truth. I didn't believe it until I looked it up in the Helms myself, but our cars "Diesel" when we decelerate in gear. It's best to down shift and use the brakes as little as possible to achieve maximum gooey gas mileage-ness

It's also good for the tranny as wel!l (providing the shifts are smooth).

cdntegintx

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Dogginator &raquo;</TD></TR><TR><TD CLASS="quote">TPS controlling the amount of fuel.

I'll be nice. So here it is. The amount of fuel fed into the cylinder is determined by the oxygen sensor during normal operation. The TPS has little to do with the actual amount of fuel fed into the engine: it serves more as a predictictor to changing fuel needs as the throttle position changes.</TD></TR></TABLE>

I'm going to disagree with this. The ECU will control the amount of fuel the injectors put into the cylinder. The TPS, O2, MAP, etc are all sensors to aid the ECU in determining how much fuel goes into the cylinder. There are times like open loop and WOT, when the O2 sensor doesn't come into play.

Dogginator

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by cdntegintx &raquo;</TD></TR><TR><TD CLASS="quote">

I'm going to disagree with this. The ECU will control the amount of fuel the injectors put into the cylinder. The TPS, O2, MAP, etc are all sensors to aid the ECU in determining how much fuel goes into the cylinder. There are times like open loop and WOT, when the O2 sensor doesn't come into play. </TD></TR></TABLE>

Did you fully read my post? I'll quote the critical part.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Dogginator &raquo;</TD></TR><TR><TD CLASS="quote">The exceptions to the oxygen sensor control is open loop (cold engine) and WOT.</TD></TR></TABLE>

In closed loop, the O2 sensor provides the critical feedback to the ECU to ensure the correct stoichiometery for the catalyst to function properly.

kweezerkwee

another topic down shifting waste gas? yes or no and how much?

nsxtasy

It's really quite simple.

For the best fuel economy, keep the car in the highest gear possible without letting it bog. That means shifting at the lowest RPM possible such that it doesn't bog after the upshift.

For the best acceleration, keep the car in the lowest gear possible without exceeding redline. That means shifting at redline. This assumes, of course, that you are keeping the accelerator floored for maximum acceleration.

Any questions?

SWE3TDUDE

Very educational post, I been going at this all wrong. I always thought shifting to lower gears to slow down was bad for the tranny =/ Thanks

mac_24_seven

iTrader: (1)

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by SWE3TDUDE &raquo;</TD></TR><TR><TD CLASS="quote">Very educational post, I been going at this all wrong. I always thought shifting to lower gears to slow down was bad for the tranny =/ Thanks </TD></TR></TABLE>

I assumed so as well. Local tranny guru type dude was talkin to me about this.

Now, it's not good if you're a n00b who can't drive and it's all jerky and stuff. I happen to pride myself on silky smooth downshifts

And, so the question isn't asked... The reason why it's beneficial to the transmission is the gears will wear somewhat more even, as pressure is being placed on them in both accelerating and decelerating, also allowing lubrication to both sides of the gears.

It made perfect sense to me and my admitted low knowledge of gears/transmissions and whatnot, so I went with it.

FW_EJ8

So if i am driving on a local street and shift at 2k everytime, it will use a bit more gas? Because if i shift into 2nd at 2k rpm wouldnt the car rpm drop low and i would have to step on the gas more to get it up again?

MrViet

so based on all this info and stuff....shifting at 2k is no good and doesn't save gas?

haha just an answer to that question will close this case

subway

Nice nice, very educational indeed! Time to start saving gas. gas prices ftl!

Dogginator

This is a plot you all need to see. It is for a JDM Si VTEC B18C. The bottom line in the plot is (g/hr)/PS. The lower that number, the greater the efficiency. Keep in mind this is at WOT.



I originally posted it here a while back.

https://honda-tech.com/zero...32914

Anyone care to explain this plot?

It is from here:
http://www.honda.co.jp/factboo....html

VTEC_PRODUCTION

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by mac_24_seven &raquo;</TD></TR><TR><TD CLASS="quote">


He speaks truth. I didn't believe it until I looked it up in the Helms myself, but our cars "Diesel" when we decelerate in gear. It's best to down shift and use the brakes as little as possible to achieve maximum gooey gas mileage-ness

It's also good for the tranny as wel!l (providing the shifts are smooth).</TD></TR></TABLE>

i dont know how its really good for the tranny, but its best to coast and brake early and softly, thats how you really get the efficiency out of it. i dont think down shifting is the best thing for the engine since it puts alot of unnecessary spin on the bearings.

DA_9

This has got to be one of the most educational posts, as well as informative, and worth while reading in a long time.