RPM @ 70 for Best MPG - JKowners.com : Jeep Wrangler JK Forum
 
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post #1 of 19 Old 04-11-2010, 08:57 AM Thread Starter
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Question RPM @ 70 for Best MPG

I just took delivery of a new 4DR Sahara. I am wondering what the ideal RPM would be for best economy when cruising 70 mph. When I had my Dodge Cummins, the ideal economy (for a diesel) was just above peak torque.

What is peak torque in this engine?

I notice it takes very little wind or hills to shift down. Even though this has the 3.73 it still seems to be gear a little to high and/or there is a huge jump from direct (3rd) to Overdrive (4th)...around 1K rpm's. Just the little I have played with it, it sure seems like 2100 or 2200 might be a better OD cruising rpm.

Thanks for any help.

Love the Jeep.

jjw
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post #2 of 19 Old 04-11-2010, 09:33 AM
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Welcome to 3.8L of fury my friend.

Gas mileage? Throw it out the window and don't watch the dash gauge because it is not accurate at all, even stock.

As far as cruising goes, just let it do what it does. The auto is going to shift in and out of overdrive if the guy in front of you farts. Give it a hill? Its gonna give it hell.

Just get used to it, they are underpowered and under geared from the factory but I promise you will get used to it. I PROMISE. We have all bitched and moaned about this engine and then we throw big tires on em and bitch some more.

Give it some time to get used to you, and you to get used to it, and you'll figure out that you can't calculate MPG accurately no matter what you do.

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post #3 of 19 Old 04-11-2010, 12:07 PM
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Youre going to be at the same RPM at 70 regardless you have no control over that lol its your gearing.....

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post #4 of 19 Old 04-11-2010, 12:14 PM
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Have your tow rig run about 2100 RPM's at 70, that should net you about 18-19 mpg if it is a diesel.


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post #5 of 19 Old 04-11-2010, 12:25 PM
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I think peak torque is somewhere around 3500. Going up hills before I regeared I would be at this range. Now I know V6's like to spin, but I tell you it's no fun listening to this engine spinning at this RPM. You swear that the thing is going to fly apart. Do as suggested, find a comfortable range and enjoy the view. Its a Jeep Baby!!
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post #6 of 19 Old 04-11-2010, 06:09 PM
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I was very happy to get 300 miles out of a tank this past week, and I'll have to put in at least 20 gallons, which makes barely 15 MPG with 35's, an auto, and 3.73's.

My recommendation would be to slow down. Wind resistance increases exponentially with speed so I suggest you slow it down to a speed between 58 and 65 at the most. I live in NY, so this is above the speed limit everywhere but the Thruway, but this keeps the RPM's around 1500 in OD and 2050 with OD off.
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post #7 of 19 Old 04-11-2010, 06:33 PM Thread Starter
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Gears to get target RPM

I am looking for those that changed gears or those that have smaller tires, different gears with the same stock 3.8. I realize I would have to change gears to change the RPM as 70. If so....what would be the target RPM at 70. Knowing this would help determine what gears one should go with.

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post #8 of 19 Old 04-12-2010, 12:00 AM
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"it's a jeep thang"


i will say that is a very ambiguos question because...

first of all, the 3.8 is a straight up, primitive engine...

like ive posted in other places, it burns too much oil, it's inneficient, underpowered, but it is, in a jeep... like i said, it's a jeep thing

first of all, the wrangler does not know what "aerodynamic...thinga...mah....jig" means, it looks like a toaster...c'mon...

id just like to make ppl realize, a wrangler is not about.......... anything but fun......


im sorry im getting all philosophical and off topic...

here's some theory

diesels get better mileage out of peak torque because that is where the threshold of horspower and tprque exists for a diesel...\

gas engine...wait...
\
gas engine plus wrangler

does it really matter????????????

you care about mileage and you get a jeep, "Just Empty Every Pocket"? sorry, no relation, the environment? ummmmmm, we go places we dont have to cause we wanto...sorry, it doesnt apply....again, the environment.... ummm, even the slowest of slow, can tell that a brick, is not aero dynamic....

what i am trying to get at is this.... you want better mileage...take 59seconds to 60mph, brake 500ft in advance, and never go above 55 (research has shown that this is the speed to keep for an average vehicle to get the best mileage and power combo)...

you want mileage.... dont get a wrangler


you want ultimate driving freedom and fun.... youre already reading!!!!
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post #9 of 19 Old 04-12-2010, 04:13 AM
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3.8l peak torque 240 lbs-ft @ 4,000 rpm.

Good luck with that.
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post #10 of 19 Old 04-12-2010, 05:40 AM
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Quote:
Originally Posted by ZOSO-Green View Post
3.8l peak torque 240 lbs-ft @ 4,000 rpm.

Good luck with that.
That was just mean.

Correct, but mean...


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post #11 of 19 Old 04-12-2010, 07:49 AM
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I have an automatic, 5.13 gears, superchips flashpaq 91 setting, 37" Iroks. Here is what I run:

70MPH on flat open road = 2400 RPM

70MPH uphill with OD off = 3500 RPM

Engine runs fine, does not get hot. Runs great uphill after gear install. No more auto tranny looking for a gear after the flashpaq install. Seems to love / perform best in the 3500 RPM range. Don't be shy about getting the RPMs up there if you need to.

Most JK owners that hate the 3.8 are first-time Jeep owners. Trust me, it runs way faster on the highway than any of the previous model of Jeeps, and gets better gas mileage. I currently have a TJ, and had a YJ, a CJ, and an XJ. The JK will outrun them all on the freeway. Its a Jeep thing, you either accept it for what it is, do an engine swap, or buy something else. This has always been the case dating back 60 years.

I see too many people jump out of their Chevy Silverados or Toyota Camrys and expect the same performance from a Jeep. If this is your expectation going in, you will be disappointed.
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post #12 of 19 Old 04-12-2010, 08:16 AM
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Here's another good read jw.........it's gets little funny at times but there is some good info sprinkled in there

https://www.jkowners.com/forum/showth...t=gearing+auto

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post #13 of 19 Old 04-12-2010, 08:45 AM
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Quote:
Originally Posted by jeeperjkj View Post
That was just mean.

Correct, but mean...
LOL Not trying to be mean, Just givin' the man the info
he asked for.

BTW, My 6sp/4.10's/35's (BFG KM2's=just shy of 34") runs around 2600 rpm @ 70,
and that gets me 15-17 mpg.

The key to better mpg in a Jeep is keep a clean air filter, no less than 28 psi
in your tires and slow down.

IMO the 3.8 feels good at 2800-3000.
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post #14 of 19 Old 04-12-2010, 09:41 AM
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u will get ur best mpg at 60 mph.. lol

70mph with 5.13s and 37s.. im at 2800rpm = 15mpg

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post #15 of 19 Old 04-12-2010, 02:46 PM Thread Starter
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Thanks You

Thanks for the info. The more I drive it, I can see low to mid 2000 RPMs as a good compromise to economy and performance.

It sure if fun to drive. Rides much smoother then the older pickup it replaced.

Thanks again.

jjw
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post #16 of 19 Old 04-12-2010, 04:42 PM
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Glad to hear you're lovin' it man! Welcome to the club!

I bought mine new in 07' (1st Jeep). I am proud to say I am a full fledged junkie!
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post #17 of 19 Old 04-12-2010, 05:37 PM
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:laugh ing2:
Quote:
Originally Posted by daspes View Post
Have your tow rig run about 2100 RPM's at 70, that should net you about 18-19 mpg if it is a diesel.
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post #18 of 19 Old 04-13-2010, 11:29 AM
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I'm going to test this out one of these weeks when I can have the jeep for my commute. I drive 50 miles each day right at about 60mph. I want to test it in 6th and 5th gears to see what difference it makes.. IRRC thats at about 2000 and 2200/2300 rpm respectively- its been a while since I've driven it.

I know this whole you got a jeep screw the gas mileage attitude, but when you go through a tank a week an extra 50-100 miles out of a tank makes a big difference. on highway/city I've been getting ~19mpg the last I checked..

its just nice to know.

~ Clam

Last edited by The Clam; 04-13-2010 at 11:52 AM.
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post #19 of 19 Old 04-13-2010, 11:47 AM
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The past two weeks I've averaged 21mpg in my stock jeep. 21.2 to be exact. How? I don't know...Driving like grandpa in my 3.73 6-spd?

I've got the 35's in the garage waiting to go on...I assume my mpg will plummet and oil stock will skyrocket as a result...

Driving like it's a piece of glass will get you the best mpg.

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