Re-geared from 2.73 to 3.54 - sounds like she's gonna blow! - JeepForum.com
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Unread 03-20-2015, 04:26 PM   #1
Colt44
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Re-geared from 2.73 to 3.54 - sounds like she's gonna blow!

Well I finally swapped out the OEM 2.73 gearing (Automatic trans came with 2.73 by default) for 3.54 and now it sounds like it's running at way too high an rpm at highway speed.

I've looked at all the gearing charts which show I should be running at about 2500 - 2600 rpm at 65 mph for a 3.54 on 31" tires, which is "in the green" on all the charts I can find online.

Is it just that I was so used to running at too low an rpm and so what is normal sounds too high to me, or should I be concerned, need bigger tires, etc?

Of course, they didn't put tachs on the auto models, so I guess I'll need to add one unless someone can tell me another way to check the rpms as I drive.

Suggestions, advice?

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Unread 03-20-2015, 04:31 PM   #2
Mr. Boh
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Try this:
http://www.grimmjeeper.com/gears.html

That should give you a real good comparison from your stock with 2.73 and 235s to 3.54 with 31s.
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Unread 03-20-2015, 05:03 PM   #3
Erik719
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Quote:
Originally Posted by Colt44 View Post

Of course, they didn't put tachs on the auto models, so I guess I'll need to add one
Think you answered it.

You could get a cheap on at the auto parts and just run the wires in through the window if all you want to do is check.
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Unread 03-20-2015, 05:05 PM   #4
skizriz
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I have 4.88's with a 3 speed auto. At 55mph it sounds like the valves are going to shoot out through the hood. lol
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Unread 03-20-2015, 07:04 PM   #5
Colt44
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According to grimmjeeper, here is what the RPM would be for each gear ratio.

So, at 65 mph I'm running 571 rpm higher now than before; no wonder it sounds so high to me now.
Obviously I was running at a too-low engine speed before, so I need to get used to "the new normal." (Apologies to Obama)
What's a normal cruising rpm for the 258-6? Is 2500 rpm at 65 mph about right?

Screenshot from grimmjeeper; obviously 3.54 numbers at top and 2.73 below.
rpms.jpg

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Last edited by Colt44; 03-20-2015 at 07:09 PM.. Reason: Formatting problems
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Unread 03-20-2015, 07:25 PM   #6
Colt44
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Quote:
Originally Posted by Erik719 View Post
Think you answered it.

You could get a cheap on at the auto parts and just run the wires in through the window if all you want to do is check.
Hmm... could I just yank one from a junkyard donor like a Wrangler or Cherokee?
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Unread 03-20-2015, 08:58 PM   #7
CoryA
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Your also assuming your 31" tires are 31" tall try actually measuring them with a tape measure and then run the calculator
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Unread 03-20-2015, 09:10 PM   #8
captkhaos
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They put tachs in Laredos with automatics. Ask me how I know.
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Unread 03-20-2015, 10:14 PM   #9
Cutlass327
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Hmm.. my old Sunpro must be off - I'm running 3k at 70MPH with 1.5 year old BFG ATs, 3.54s, and T177.
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Unread 03-20-2015, 11:23 PM   #10
jeepdaddy2000
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Buy a tach. You can't do gearing work without one.

Quote:
Originally Posted by Cutlass327 View Post
Hmm.. my old Sunpro must be off - I'm running 3k at 70MPH with 1.5 year old BFG ATs, 3.54s, and T177.
Most universal aftermarket tachs have separate settings for 4, 6, and 8 cylinder motors. The rear cover needs to be taken off to access the switch. If the switch isn't in the right place, the tatch won't read the proper RPM's (you probably knew this).
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Unread 03-21-2015, 05:54 AM   #11
Mike Romain
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The sweet spot for the 258 is around 2300 rpm. You will drink a lot of gas turning 2600+.
For sure those gears could take 33's.
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89 YJ Renegade. BBD Carbed 4.0 HO. Locked front and rear with 33x9.5 BFG AT's
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Unread 03-21-2015, 07:18 AM   #12
HackFabrication
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Quote:
Originally Posted by Colt44 View Post
Suggestions, advice?
Turn up the stereo.....
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Unread 03-21-2015, 07:22 AM   #13
swatson454
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Mine sounded like an old prop plane ready to take flight until I installed a fan clutch.


Shawn
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Unread 03-21-2015, 07:30 AM   #14
Mike Romain
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Mine sounded like an old prop plane ready to take flight until I installed a fan clutch.

Shawn
Mine has the fixed fan and yes she sure roars.
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89 YJ Renegade. BBD Carbed 4.0 HO. Locked front and rear with 33x9.5 BFG AT's
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Unread 03-21-2015, 07:44 AM   #15
WindKnot
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Think of it as loud pipes on a motorcycle! Mine roars too.
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