258 RPMs & Gear Ratios - JeepForum.com
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post #1 of 13 Old 06-27-2020, 10:28 PM Thread Starter
Flundertaker
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258 RPMs & Gear Ratios

So my 86 has a 258, T5, 3.54 gears in the diffs, 31" tires. What sort of operating range RPMs should this drivetrain run? Redline?

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post #2 of 13 Old 06-28-2020, 06:21 AM
shadoow
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post #3 of 13 Old 06-28-2020, 09:47 AM Thread Starter
Flundertaker
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Quote:
Originally Posted by shadoow View Post

Thanks. Itís helpful but doesnít tell me anything about where my 258 likes to live and where itís most powerful or efficient.


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post #4 of 13 Old 06-28-2020, 10:08 AM
oldschool74cj5
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hello

the 258 is pretty much a low rpm tractor motor. it is a undersquare engine meaning that the bore is smaller then the stroke. with that said that is what makes her such a torque motor for low rpm but stays pretty flat. peak torque of the motor is around 1800-2000. the max hp is around 3500 rpm with most cams. its the torque that does the work. they can actually measure the torque but have to calculate the HP. i cruise between the 2000 and 2500 rpm. with out being on the gas much. i have alot of pedal left and she will pull to about 3800 then she starts wheezing. to me above 4k is pretty much useless. my gearing ratio is close to what you have. i am running 3.73 with 33" tires. but im also not in the mountains. im in FL. i have taken her to the mountains and it does well.

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post #5 of 13 Old 07-06-2020, 11:14 PM
Nucking1Futs
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Quote:
Originally Posted by Flundertaker View Post
Thanks. Itís helpful but doesnít tell me anything about where my 258 likes to live and where itís most powerful or efficient.


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I made a chart with the estimated HP and torque potential of a new factory 1984 4.2L (258) engine using data from
https://www.automobile-catalog.com/c...l_5-speed.html

I do not know the accuracy of the data, but seems pretty close. Assuming the data is a good representation to the truth, this is a best case scenario to ballpark (visually) what is going on. I found it to be enlightening myself (visual learner I suppose). You can see why everyone keeps preaching (and correctly) about lower gears. CJ5/6/7/8 are tools, utilitarian, built to cover most driving conditions but shifted to include the non-pavement side of driving. As yo see from GrimmJeeper website, who can hone in your jeep to fit your needs.

Others who may have better information on HP and Torque curve if it is truly different can weigh in to fine tune it. Plug in your data into Excel (numbers for Mac) with separate columns for RPM, HP, Torque and make a scatter plot.Easy Peasy!
Good luck and have fun.
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4.2L w/Howell TBI, T5, D300, Dana 30 w/locker/AMC 20 posit, 3.31 gears
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post #6 of 13 Old 07-07-2020, 04:23 AM
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If you want fuel economy, drive slowly, no kidding, but at a point where you are not needing to mobilise all the available torque to maintain a good speed. Engines of this era were designed to give best economy on partial throttle. if your foot is buried in the carpet (yeah, if you have any) to try and and get it to go, you are going to drink fuel. This is sometimes because the gearing means the revs are too high, or too low. Mostly it is just because the Jeep was built to have peak torque at around where it equalled the forces fighting it.

Anything much over 2300 and peak torque and I am going too fast in top gear (1:1) for economy and responsiveness.

My J20 has a 258, as do 2 of my CJ7s, and this holds for all of them. Due to gearing my J20 is much happier at slower speeds than the CJ7s but it has the torque to carry a heavy load at that speed.

My XJ Cherokee has a 4.0 HO which breathes better and has more torque higher up, it falls off after 3800 so that is where I am happiest taking it. Maximum torque is only 10% higher than a 258 but it is there at much higher rpms, so the peak horsepower is also much higher. (Do the maths 3800/2300 x 1.1 - 1 = 60% more peak power). it is a rocket ship by comparison.

available Horsepower = Available Torque x rpm

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1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
1995 Cherokee 4.0 - CHEROKEE
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post #7 of 13 Old 07-07-2020, 02:35 PM
LumpyGrits
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No '86 6cyl had as OEM 3:54 R&P.

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post #8 of 13 Old 07-07-2020, 02:37 PM
LumpyGrits
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258 redline is 4500rpm
Cruise RPM 2200-2400 is good.
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post #9 of 13 Old 07-08-2020, 04:38 AM
keith460
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Quote:
Originally Posted by LumpyGrits View Post
No '86 6cyl had as OEM 3:54 R&P.

Not for '86. Standard gears were the dreaded 2:73 with 3:31 as an option. 3:54 was standard for the 4 cylinder engine and 4:09 being the option.

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post #10 of 13 Old 07-08-2020, 05:24 AM
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Quote:
Originally Posted by LumpyGrits View Post
Cruise RPM 2200-2400 is good.
At what speed?

Remember, the faster you go the higher the air resistance. "Cruise" for some may be 55MPH while others like 70+.

Terrain also takes a bite out of the equation. I live in Southern Oregon. Going North requires me to go through four passes to get to Roseburg. Lower gearing for me means keeping my power up climbing these grades, something a person living in Texas or Nebraska won't have to worry about.

Taller gearing may mean going deeper into the carb to maintain certain speeds. Too low RPM's can cause you to run on the power circuit, wasting fuel not saving it.

Personally, I like something in the 2400-2700RPM range @60MPH for the terrain I drive.

Quote:
So my 86 has a 258, T5, 3.54 gears in the diffs, 31" tires. What sort of operating range RPMs should this drivetrain run?
So the real question is how does your rig currently feel to you?
What current RPM's are you running at 60MPH?

Are you thinking of any changes to the tire size?
Lift?
How is first gear?

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post #11 of 13 Old 07-08-2020, 06:23 AM
LumpyGrits
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2200-2400 RPM works well on the f'way.
I shift at 2500-2600RPM.
The freeway speed is dependent on your R&P, W/tire size and if your tranny has OD or not.
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post #12 of 13 Old 07-08-2020, 07:49 AM
John Strenk
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A vacuum gauge is a pretty good way to see the load on your engine at any given speed.

Hook one up to your intake manifold and try to drive at the highest vacuum numbers you can maintain.

Volkswagen had determined a formula to determine the fuel economy on any vehicle. All you needed was to know your rolling resistance, air resistance, and a couple other things that you can find out about on any vehicle with some simple experimentation. Once you figure out how much HP is eaten up with rolling resistance and air resistance you can find out how much available HP you have left over to maintain a set speed.

The more available HP the more economical. If the RPM at your desired speed doesn't give you enough HP you can change gearing to select the right RPM to give you maximum economy.

I once had an excel worksheet that took all those parameters in but I don't know what happened to it. But it was interesting to see how much HP was needed to keep a CJ-7 with 2.73 gears on 33" tires. There was insufficient HP available and it looked like the CJ would actually slow down until the HP demands and available HP were the same.


Here is another fun way to determines an engines best RPM for economy:
https://x-engineer.org/automotive-en...sumption-bsfc/


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post #13 of 13 Old 07-08-2020, 07:42 PM Thread Starter
Flundertaker
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OP here

couple things,,,,,
1. my mistake, R&P is 3.31 43 13
2. Tires are 31x10.50x15
3. Trans is T5

I had a freeway drive today my speedo is 5 mph slow based on my GPS due to tire size. The roadway was concrete and the expansion joints made the drive miserable. 55-60 mph is about as fast I could safely go due to the bucking back and forth. My tach and Sniper readout both read 1850-2100 rpms.
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