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Old 08-26-2011, 09:03 AM   #1
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Optimum rate of acceleration

Seeing that I just got my TJ Unlimited, I'm trying to determine the optimum rate of acceleration when getting up to speed out of a stop light. It appears to be that while flooring it is not good for gas mileage, neither is gently, barely getting up to speed. I've seen this quote online:

"Every vehicle has a rate of acceleration that provides maximum possible efficiency. deviating from that in EITHER direction reduces efficiency."

Does anyone driving this Wrangler know what that optimum rate of acceleration is in order to get the optimum city MPG? It seems to be that gently getting up to speed keeps the motor running at an "above cruising" RPM for a longer period, whereas if I just simply got up to speed at 2k and then the motor drops down to a lower RPM in the low spots for a given gear SOONER, would have the motor running at lower RPMs LONGER given an hour of city driving. I guess I would have to be looking at "how many revolutions are turning over how many feet for how long" for every stretch of the route.

I'm asking because I'm getting well below 15 MPG on a barely-broken in Jeep that runs good. I had been driving it gently on the acceleration rate, and maybe my thinking had been wrong.

Thanks, everyone

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Old 08-26-2011, 10:18 AM   #2
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Well at least in my thinking, though I don't give too much thought to gas mileage on our Jeep. Let's see, flog it to get up to speed would seem to be wasteful in the fuel needed to run hard to get there. On the other hand, she wore a glove. Whoops wrong story.

A turtle pace, on the other hand, I could see maybe a couple of problems. One if it's on a road with stop lights, you may not even get up to speed before having to stop, and two, not to mention getting run over by the folks behind you.

And the third thing is no two jeeps are alike, what may work for me, may not work for you. You have to think about rolling resistance, not only from it being a 4WD, but also the tires themselves, the tire size in relation to the gearing, tire pressure for fuel mileage vs ride, engine size, a/c on or off.

Way too many things for me to worry about. But then again I think about how I drive, with what I have. I don't jack rabbit start, I don't turtle either. Steady acceleration, and quick off the gas when I see I am going to see that I will be needing to slow down and/or stop. Something I learned driving big rigs, look way ahead when you are driving.

That alone saves gas. I'm not one of those who waits till the last minute to hit the brakes. I'm rolling in at idle and then lightly applying the brakes. I mean what's the deal with racing to a light just so you can stop and sit.

Also I tend to run below the speed limit, especially on the big highways. At 70 mph the jeep is working hard, drop to 65 or 60, and less work for the engine. I also leave early so I'm not one of those idiots zipping in and out of traffic, cause they slept in.

They may beat me to a place by a minute or two, but funny I usually glide up to them at the stop light, where they've been sitting, and I don't have to stop as the light has now changed. So there I'm already rolling, no initial throttle down needed.

Little things that add up. I run a taller tire, it goes further per revolution, but I didn't go bonkers on the size so that the engine is straining to turn em due to the gear ratio.

And yes I'm one of those who can squeeze nearly 20 mpg city/highway combined. But I was taught how to be frugal on the go pedal, and to look at more then it to get good mileage. Oh and I run an automatic trans, 4.0.

The wifey, well she was raised in Pittsburgh, PA. Where it's mostly city driving by nut cases. She scares the bajeezus out of me when she drives. On the gas, off the gas, on the brake, off the brake, one foot on the gas, one on the brake. And she's one of those nut cases weaving in and out of traffic. She can though give lessons on drafting. I've never seen the back side of a semi so close. For her maybe 8 to 10 mpg on a good day.

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Old 08-26-2011, 10:42 AM   #3
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Shelby,

Those are good hints and observations, especially about rolling up beside someone who shoots out ahead at the previous light with their farty exhaust noise. I follow the big rig principles when it comes to judging what is happening up to two lights down and plotting a "rolling path." I did this even when I had a sedan. I try to watch on the highway what someone several cars up is doing. As soon as that someone hits the brakes, I back off the accelerator.
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Old 08-26-2011, 10:50 AM   #4
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In my experience my mpgs don't change whether I beat on it and floor it, or take it very easy. I suggest just driving normally and not too hard, and keeping rpms slightly about 2k. This isn't easy in an auto, in the city.
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Old 08-26-2011, 01:54 PM   #5
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Don't floor it, because then the ECU will run rich, in open-loop mode. And this will use more gas.

My guess is if you shift around 1800 RPM or so, and keep under 75% throttle, you'll do best. At that low of engine speed a throttle opening of more than 2/3 or so will probably not make any difference. This is for the 4.0L, by the way.
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Old 08-26-2011, 01:56 PM   #6
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Its true, it doesn't. because If I smash it to the floor it goes from 1800 to 2300...
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Old 08-26-2011, 04:01 PM   #7
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Quote:
Originally Posted by AC0QR View Post
My guess is if you shift around 1800 RPM or so, and keep under 75% throttle, you'll do best. At that low of engine speed a throttle opening of more than 2/3 or so will probably not make any difference. This is for the 4.0L, by the way.
in 1st w/my 4 banger-well, i could walk faster. i'm usually anywhere in the 2.5-3K range. this usually gives a smooth shift and good accerlation.
like mentioned above, driving habits have alot to do with it-quick starts/stops and hi rpm usually don't do much for mileage. i get anywhere between 17-21 depending on how well i behave my self.
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Old 08-26-2011, 04:10 PM   #8
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Quote:
Originally Posted by GoldenSahara00
In my experience my mpgs don't change whether I beat on it and floor it, or take it very easy. I suggest just driving normally and not too hard, and keeping rpms slightly about 2k. This isn't easy in an auto, in the city.
I just don't understand how! Did you ever change out your speedometer gear?

I can knock mine down to 12mpg if I get on the skinny pedal and let it wind out a bit or if I drive conservatively I can hit 18mpg.
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Old 08-26-2011, 04:22 PM   #9
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You could buy a Scanguage II. It's helped me monitor my gas usage, and has helped me improve my MPG with the info it provides. Also, what size tire are you running, tranny, and gearing in the diffs?
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Old 08-26-2011, 04:25 PM   #10
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Quote:
Originally Posted by InvertChaos

I just don't understand how! Did you ever change out your speedometer gear?

I can knock mine down to 12mpg if I get on the skinny pedal and let it wind out a bit or if I drive conservatively I can hit 18mpg.
If I drive aggressively on the highway I drop down to 15-16 mpg. If I stay calm and obey the speed limit, I've hit 19 before. My driving habits drastically change my gas mileage too. I also watch my fuel mileage very carefully.
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Old 08-26-2011, 10:42 PM   #11
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Nope but I have calculated my average miles driven with actuall miles according to GPS then averaged those (which were all within a hundredth of eachother) and then applied that to my mileage to calculate the correct mileage. Something other than my speedo gear is off because they were off before I got new tires... I am not sure what.

But I got 22 mpgs the summer I bought my jeep when it was on 235s doorless and topless no rear seat. OH YEA.

I burnt 1/4 of a tank on the highway in about 15 mins tonite trying to get home fast... lol that was def not gas efficient. As long as I run between 45-55 mph I seem to get good mpgs.
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Old 08-26-2011, 10:51 PM   #12
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i usually floor it in first until about 4.5k, then shift to second. I don't let up on the gas while I shift however, so once 2nd engages my wheels give the person next to me confirmation of what they think they are hearing from my engine. I don't use gears 3-5.
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Old 08-26-2011, 11:04 PM   #13
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interesting.
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Old 08-26-2011, 11:11 PM   #14
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Quote:
Originally Posted by jaank
i usually floor it in first until about 4.5k, then shift to second. I don't let up on the gas while I shift however, so once 2nd engages my wheels give the person next to me confirmation of what they think they are hearing from my engine. I don't use gears 3-5.
Video or it didn't happen
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Old 08-27-2011, 12:39 AM   #15
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Idk but I get awful gas mileage too. On a good day 12-13. And I don't run over 2000 rpm at any one time. I have an 3speed auto with 3.08's and 32" curious how your all getting 15+ on a bad day
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Old 08-27-2011, 01:15 AM   #16
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in all seriousness in my 2000 TJ I am not very gentle, shift around 3k, don't go to 5th unless exceeding 45mph, and am pretty generous with the skinny pedal, and I get 14-15mpg, almost all in town driving.
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Old 08-27-2011, 07:33 AM   #17
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Quote:
Originally Posted by jaank
in all seriousness in my 2000 TJ I am not very gentle, shift around 3k, don't go to 5th unless exceeding 45mph, and am pretty generous with the skinny pedal, and I get 14-15mpg, almost all in town driving.
Darn I was hoping to see a video of you hitting 70mph in 2nd lol
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Old 08-27-2011, 09:10 AM   #18
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Don't floor it, because then the ECU will run rich, in open-loop mode. And this will use more gas.
Yep, mine runs rich 90% of the time. I am 45 going on 16.
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Old 08-27-2011, 10:19 AM   #19
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I can do 65 in 2nd...

mike kaz, You have the 4.0? are you calculating your mpgs according to your tire sizez change?
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Old 08-27-2011, 01:18 PM   #20
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I can do 65 in 2nd...

mike kaz, You have the 4.0? are you calculating your mpgs according to your tire sizez change?
Yes I have the 4.0. I'm calculating by the miles per tank I'm getting and then dividing it down to gallon
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Old 08-27-2011, 01:23 PM   #21
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you need to realise that your odometer is off by a certain percentage because you went from 28 to 32 inch tires.
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Old 08-27-2011, 02:21 PM   #22
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Yep if your speedo isn't right, your odo isn't either. You'll be getting better gas mileage than you think because you are traveling further than your odo says.
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Old 08-27-2011, 03:00 PM   #23
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Yep if your speedo isn't right, your odo isn't either. You'll be getting better gas mileage than you think because you are traveling further than your odo says.
Yup, most people don't realize that. I get over 18 mpg average with my 4.0, auto with 33s. I drive very slowly though and probably piss a lot of people off that get stuck behind me at a stop. I actually do a little better than that because I have not changed the gear on my speedo so I am going farther that what the numbers say.
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Old 08-27-2011, 03:10 PM   #24
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It's been a while since my Newtonian physics course, but I'm fairly certain the faster you accelerate the more power is needed.

Force = Mass * Acceleration

Your mass is constant. As acceleration goes up, the force is increased. Since the force your engine outputs comes from the energy in gas, you would burn more gas.
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Old 08-27-2011, 04:36 PM   #25
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Originally Posted by GoldenSahara00
you need to realise that your odometer is off by a certain percentage because you went from 28 to 32 inch tires.
Well my lil sticker on the door jam said it came with 30" wheels. (Sahara edition thing idk?). So the 2" shouldn't be that drastic of a mph drop.
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Old 08-27-2011, 04:57 PM   #26
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Worrying about what is the optimal acceleration rate for a Jeep?

Shakespeare expressed my thoughts precisely when he penned the comedic play "Much ado about nothing". Analysis paralysis comes to mind too.
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Old 08-27-2011, 05:04 PM   #27
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It's been a while since my Newtonian physics course, but I'm fairly certain the faster you accelerate the more power is needed.

Force = Mass * Acceleration

Your mass is constant. As acceleration goes up, the force is increased. Since the force your engine outputs comes from the energy in gas, you would burn more gas.
True, and your engine has different power levels at different rpm's which means it works less at certain rpms. And the quicker you get to that certain speed, the sooner you can ease off the skinny pedal. Definately not cost effective to floor it every gear, but just the same, its not cost effective to baby it either from gear to gear. Like someone said already, no matter how I do it in my Jeep, I get about the same fuel economy, or lack of. Now in my Mustang, I can really tell the difference in my pocket. My Jeep with 33's is about 1 mile off every 10 miles. Meaning when my odometer clicks nine miles the vehicle has traveled approximately 10.
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Old 08-27-2011, 05:13 PM   #28
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i get best gas mileage at around 1.21 jiggawatts... but thats just my jeep
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Old 08-27-2011, 07:16 PM   #29
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i get best gas mileage at around 1.21 jiggawatts... but thats just my jeep
Oh yeah!
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Old 08-27-2011, 09:43 PM   #30
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Well my lil sticker on the door jam said it came with 30" wheels. (Sahara edition thing idk?). So the 2" shouldn't be that drastic of a mph drop.
roughly 6%. so you get about 1 mpg better than you think. roughly.

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