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Old 05-09-2011, 04:57 PM   #1
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manual - shift speed - mpg impact?

If you want to maximize fuel economy, do you shift at 2500 RPM? 3000? Higher?

I've been shifting low (2500 rpm) assuming that the lower the engine spins the greater the fuel economy. That means that I'm always down on the power curve so acceleration requires a downshift.

I've not yet collected enough mpg data to know if I'm benefitting from the practice.

- Keith -

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Old 05-09-2011, 05:17 PM   #2
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the lower the rpm the better for mpg. i shift about 2300-2500 into 2nd and try to keep it below 2000 for the rest of the gears.

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Old 05-09-2011, 05:21 PM   #3
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Not with the 3.8L !!


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the lower the rpm the better for mpg. i shift about 2300-2500 into 2nd and try to keep it below 2000 for the rest of the gears.
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Old 05-09-2011, 05:25 PM   #4
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Not with the 3.8L !!
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Sooo, tell me more ...

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Old 05-09-2011, 05:45 PM   #5
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well, the 3.8" doesn't haqve an abundance of torque in the lower rpms (800/2000, so


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Sooo, tell me more ...

- Keith -
If you can keep it (hiwy travel) between 2400 and 2700 rpm (withouit shifting), everytime you shift DOWN, you'll lose a % of your mileage !!

I have the auto and for 1˝ yrs with big tires and cruising at 65/2200 rpm, my mileage avg was 15.2 mpg--of course other speeds involved !

Now while I'm still in the "breakin" mode and I cruise at 65/2500 rpm and my mileage is avg 16.9 mpg-

It don't make a lotta sense, but it's happening right here in front ofus !!

Keepyerpowderdry

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Old 05-09-2011, 05:47 PM   #6
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I have found that if my acceleration is the same it doesn't matter if I shift at 2k or 2700. My thought is, shifting at low rpms makes the engine work harder, especially in higher gears. At high rpms the engine doesnt work as hard, but uses more gas.
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Old 05-09-2011, 05:50 PM   #7
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I have found that if my acceleration is the same it doesn't matter if I shift at 2k or 2700. My thought is, shifting at low rpms makes the engine work harder, especially in higher gears. At high rpms the engine doesnt work as hard, but uses more gas.
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Old 05-09-2011, 06:04 PM   #8
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With the 3.8, you've got to balance power and efficiency. Here's the rule I've observed: Go easy on the gas pedal.

Sounds simple, but here's what I mean.

- If you need acceleration, shift into a gear that allows you to rev the engine up (~2500 to ~3500) while being reasonably light on the gas pedal.

- If you don't need acceleration but are simply maintaining speed, shift to a gear that allows you to keep RPMs low while still being reasonably light on the gas pedal.

Driven this way, you'll getting the most power per gallon, and, therefore, should use less fuel.

Mileage killers are (a) high revs plus a lot of gas or (b) low revs plus a lot of gas. The first one is obvious (albeit necessary when a lot of acceleration is required), but the second one fools folks. Low RPMs can kill your mileage if you're flooring the gas pedal--you're "lugging" the engine.
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Old 05-09-2011, 06:19 PM   #9
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I shift before 3,000 rpm unless I'm passing someone ect...
I estimate 2,700 or so is typical.
I have a 4 door Unlimited and nearly always get a calculated 18+ mpg driving to / from work on rural roads with wife & kids on board; the optimistic display reports an inaccurate 19+ mpg.

Said another way, I've got almost 5,000 miles on the Jeep and the worst mileage I got was 17.8 and that included several miles of 4WD on snowy roads.

It's stock and has 3.73 gears.
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Old 05-09-2011, 06:30 PM   #10
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The problem you can run into when you shift to early is the higher gear drops the rpm so fast and the hp at the lower rpms is so low it strains to keep momentum which takes more fuel. So there is an optimum, just not sure where it is.
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Old 05-09-2011, 06:39 PM   #11
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94' YJ

I shift to second at about 23- 2500.
Second to third at about 27-3.
20- 25 ish the rest
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Old 05-09-2011, 08:04 PM   #12
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Everyone above is just about correct. You can figure out intuitively the two or three that are just plain wrong.

Real world example: I shift at the lowest possible RPM to the next gear when I know it will be smooth and there will be no engine strain or over-revving, both up and down. If you can hear the engine, you waited too long to shift. I get + / - 26 highway and 22 to 23 city with my 2011 Wrangler 6 Spd with 3.21s and tire inflation at + 3 cold. BTW, my '06 Corvette got 32 mpg driving it that way, but then again the Vette's engine never really worked hard at any speed or gear.

Now, on the flip side, is the way my 19 year old stirs the transmission. He has been driving manuals since he was 13, so he knows how to work a gearbox. He gets 18 highway and 14 city in my Wrangler. Why? Because he rarely upshifts at lower than 3500 RPMs and rarely uses 5th, let alone 6th.

So, within the given economy range capability of the vehicle, the resulting MPGs are at least 80% driver controlled, with the rest being terrain, temperature and actual number of stops and starts.

That's really all there is to it.

Of course corn gas vs real Dino fuel is a whole different discussion.
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Old 05-09-2011, 08:57 PM   #13
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So, the consensus is shift as early as you can without lugging the engine?

That is basically what I'm doing. The MPG computer is saying 18 - 19 over my first 100 miles of short-trip country driving. I won't know what I'm really getting until my next fill-up however.

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Old 05-09-2011, 09:14 PM   #14
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Quote:
Originally Posted by JIMBOX View Post
Not with the 3.8L !!




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lucky me i have a 4.0 with 4.56(?) gears, good old tj's.


p.s. i know this is the jk forum but i was bored and just wanted to post
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Old 05-09-2011, 11:44 PM   #15
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Quote:
Originally Posted by Celtic View Post
I get + / - 26 highway and 22 to 23 city with my 2011 Wrangler 6 Spd with 3.21s and tire inflation at + 3 cold.
Are you sure about these numbers? Not saying you're lying, but I find this very hard to believe no matter what your driving and shifting style may be.
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Old 05-10-2011, 07:12 AM   #16
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Wow, now i figure out why my fuel economy sucks so bad. I rarely shift above 2k, thinking it isn't good for the engine. Seriously, I've been shifting around1500 rpm. My fuel economy is terrible, I never go WOT either. Definitely a granny driver, babying the jeep isn't really a benefit I guess.
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Old 05-10-2011, 07:16 AM   #17
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Daedalus,
Those are the numbers coming off the DIC. They were not that high before the drive train broke in at about 3800 miles. I have been told the DIC can be 1 to 2 MPG on the optimistic side.

Also, from a stop I try to use the BRS technique as much as situations allow. And, I use 1st gear just to get it rolling with a little momentum. My 19 year old tells me I baby it. He is probably correct, but that's why I get pretty good MPGs.

The next time you are driving a vehicle with a six speed automatic transmission and a tach "on a closed course" try this:

On level terrain, keeping your foot off the gas
pedal, put it in drive and note the RPM and vehicle speed at the shift pointl from 1st to second. Then repeat the same but use half throttle until it shifts.

You should intuitively know that you used more fuel the second time than the first.

Soooooo........I'll make it real simple with two visuals:

A fresh egg that you don't break between your foot and the gas pedal or a concrete block on top of your foot.
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Old 05-10-2011, 07:21 AM   #18
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With the stock 3.21 gears I get better mileage on the highway at over 100km/hr 60mph) in 5th than in 6th...
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Old 05-10-2011, 08:44 AM   #19
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Quote:
Originally Posted by jk wrangler fan
Wow, now i figure out why my fuel economy sucks so bad. I rarely shift above 2k, thinking it isn't good for the engine. Seriously, I've been shifting around1500 rpm. My fuel economy is terrible, I never go WOT either. Definitely a granny driver, babying the jeep isn't really a benefit I guess.
Yep, lugging the engine kills mpgs. To exaggerate the point: If you always shifted into 5th gear at 20 mph or below for all your city driving, you'd be standing on the pedal all the time and your gas mileage would be terrible.

You've got to shift based on need. If you want to maximize mpgs, take that further and shift to whatever gear allows you to be lightest on the gas pedal to accomplish what you need to do, regardless of RPM. (Presuming "what you need to do" is reasonable of course.) That maximizes engine efficiency and produces the best results.

Quote:
Originally Posted by SeaComms
With the stock 3.21 gears I get better mileage on the highway at over 100km/hr 60mph) in 5th than in 6th...
That's generally my experience too--lugging kills mpgs.

If you've got to go up any kind of grade on the highway or want to maintain a speed over 70 mph even on a flat grade, you're better off downshifting to 5th rather than flooring the pedal in 6th to get up the hill or keep it up to speed against the wind. Especially with the 3.21s.

Still though, the best highway mpgs will be accomplished in 6th on a flat grade while only trying to keep up at about 65 mph. The problem is that's a pretty rare scenario.
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Old 05-10-2011, 09:51 PM   #20
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Quote:
Originally Posted by im-ocd View Post
Said another way, I've got almost 5,000 miles on the Jeep and the worst mileage I got was 17.8 and that included several miles of 4WD on snowy roads.
It's stock and has 3.73 gears.
Update (new low) my last tank included more "urban" driving than usual and I pulled my 5x8 utility trailer with a light load for 20 miles+- I only got 17.3 mpg calculated average
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Old 05-10-2011, 10:47 PM   #21
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I shift between 2500 and 3k man. But going downhill ill shift at much lower rpms.. I dont really know I've wondered about this question for a long time Ive just been experimenting and for my conclusion I write that JIMBOX is right. I found that the engine doesnt work as hard when shifting from a higher rpm because once in the new gear if your starting out above or around 2000rpm you dont lug trying to maintain speed or speed up even more say in 6th from 5th... its much smoother and seems to work the engine a lot less. But if you shift at too low of an rpm you find yourself accelerating from an rpm thats too low and it seems to struggle... I shift about 2700 and after completing the shift im at around 2200 in 6th and smoothly kick it up to around 2500 for 65 mph. But if i shift at too low of an rpm and end up at like 1500 rpms now ive rly got to work to get back up to around 2500 for highway speed.. kinda like taking off in third gear from a stop. That wasn't the most scientific explanation but thats how I think of it and thats the only way I can put it to words. Its kind of like mowing... the mower works less at a higher rpm.. if you lug it down and it struggles through the grass it sucks the gas haha
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Old 05-11-2011, 05:34 AM   #22
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... I found that the engine doesnt work as hard when shifting from a higher rpm ...
Here's why I'm not so quick to buy it.

Automatics. They seem to be all about keeping RPMs low.

The benefit of the juice tranny is that it can shift much quicker and much more frequently than most of us manual tranny users can. So the computer that is making shift decisions is free to pick the higher gear to save on fuel and then will quickly downshift to provide power when needed.

Consider this "thought experiment". Let's assume 2 identical Wranglers, traveling the same speed with the same throttle pressure; one at 2000 RPM and the other at 3000 RPM. With the same throttle pressure the fuel flow to each cylinder at each cycle will be the same but the one at the higher RPM will go through 50% more cycles and will consume 50% more fuel. But at 3000 RPM the engine will generate more power so you'll need to back off on the fuel flow to get it to the same power output as the other at 2000 (i.e. we're driving constant speed) so that will consume less fuel.

So much for "though experiments"!

A book I read many years ago about fuel economy had a very simple formula in it: Shift early, go easy on the gas, coast a lot, don't lug the engine.

- Keith -
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Old 05-11-2011, 07:18 AM   #23
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Dont lug the engine is a hard one with the 3.21 gears. At 100km/hr on the highway (60mph) driving a vehicle with worse aerodynamics than a brick I am sitting at 1750 rpm. Hardly anywhere near optimal power. Even in 5th I am only sitting around 2200rpm.

I have noticed my best fuel economy though is in 6th gear, but when sitting at as low a speed as it will still drive, ie, around 60km/hr with no wind on flat ground (had to travel for bout 50km through 60km/hr roadworks on the highway). This resulted in 8.1L/100km, or 29mpg.

I normally average 19 to 23 mpg.
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Old 05-11-2011, 08:55 AM   #24
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Here's why I'm not so quick to buy it.

Automatics. They seem to be all about keeping RPMs low.

The benefit of the juice tranny is that it can shift much quicker and much more frequently than most of us manual tranny users can. So the computer that is making shift decisions is free to pick the higher gear to save on fuel and then will quickly downshift to provide power when needed.

Consider this "thought experiment". Let's assume 2 identical Wranglers, traveling the same speed with the same throttle pressure; one at 2000 RPM and the other at 3000 RPM. With the same throttle pressure the fuel flow to each cylinder at each cycle will be the same but the one at the higher RPM will go through 50% more cycles and will consume 50% more fuel. But at 3000 RPM the engine will generate more power so you'll need to back off on the fuel flow to get it to the same power output as the other at 2000 (i.e. we're driving constant speed) so that will consume less fuel.

So much for "though experiments"!

A book I read many years ago about fuel economy had a very simple formula in it: Shift early, go easy on the gas, coast a lot, don't lug the engine.

- Keith -

automatic drivers dont have the luxury of shifting at higher rpms and with big tires they cant get as good of mpgs as us. A lot of guys chip their automatic jeeps to compensate.
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Old 05-12-2011, 08:36 PM   #25
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I shift between 2500 and 3k man. But going downhill ill shift at much lower rpms.. I dont really know I've wondered about this question for a long time Ive just been experimenting and for my conclusion I write that JIMBOX is right. I found that the engine doesnt work as hard when shifting from a higher rpm because once in the new gear if your starting out above or around 2000rpm you dont lug trying to maintain speed or speed up even more say in 6th from 5th... its much smoother and seems to work the engine a lot less. But if you shift at too low of an rpm you find yourself accelerating from an rpm thats too low and it seems to struggle... I shift about 2700 and after completing the shift im at around 2200 in 6th and smoothly kick it up to around 2500 for 65 mph. But if i shift at too low of an rpm and end up at like 1500 rpms now ive rly got to work to get back up to around 2500 for highway speed.. kinda like taking off in third gear from a stop. That wasn't the most scientific explanation but thats how I think of it and thats the only way I can put it to words. Its kind of like mowing... the mower works less at a higher rpm.. if you lug it down and it struggles through the grass it sucks the gas haha
Spot on with this strategy/explanation Man!!
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Old 05-14-2011, 04:22 PM   #26
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Old 05-14-2011, 04:28 PM   #27
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whoda thunk ??

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