Ideal Cruising RPM for 4.0 from Gearing - JeepForum.com
Search  
Sign Up   Today's Posts
User: Pass: Remember?
Advertise Here
Jeep Home Jeep Forum Jeep Classifieds Jeep Registry JeepSpace Jeep Reviews Jeep Gallery Jeep Clubs Jeep Groups Jeep Videos Jeep Events Jeep Articles
Go Back JeepForum.com > Models > Jeep Wrangler Forums > TJ Wrangler Technical Forum > Ideal Cruising RPM for 4.0 from Gearing

Labor Day Sale!FS: 2007-2013 Jeep Wrangler "HALO" Angel Eye KitEngo winches available at www.rockridge4wd.com! Free shipp

Reply
Unread 07-10-2010, 10:33 PM   #1
1C41N0
Registered User
2006 LJ Wrangler 
 
Join Date: Jan 2009
Location: South Dakota (military), Nebraska
Posts: 147
Ideal Cruising RPM for 4.0 from Gearing

I'm looking into regearing my LJ. I am wondering what is the best cruising RPM that I should shoot for. Mainly mine is a DD and will see hunting trails. I do a fair amount of freeway driving as well. I'm running the 42 RLE auto (.69 final OD gear) on 305x55R20 (33x12's). OD is about useless as it can barely keep itself in on a flat. Current diff ratio is 3.73, and I"m aiming at a cruise MPH of 70. I'm thinking around 2200 to 2300 rpms for OD. Any input is appreciated!

1C41N0 is offline   Reply With Quote
Unread 07-11-2010, 07:13 AM   #2
VnutZ
Member
 
VnutZ's Avatar
2006 LJ Wrangler 
 
Join Date: Jun 2004
Location: , Maryland
Posts: 448
Target 2700-2800. That's where I was sit on 35 (4.56) and it runs great. I've been on the full gamut of tire/gear combinations and this has proven the most ideal for me as a highway DD.
__________________
2006 Black Unlimited on Cummins 4BT
31.5 Gallon Genright Safari Tank (~900 mile range)
4" Long Arm & OME Shocks on 35x12.5" BFG KO
Ox Lockers and Warn Powerplant Winch
Improve MPG: The Factors Affecting Fuel Efficiency
Cummins 4BT Jeep Wrangler TJ Diesel Conversion
VnutZ is offline   Reply With Quote
Unread 07-11-2010, 07:24 AM   #3
lostatsea346
Registered User
2006 LJ Wrangler 
 
Join Date: Nov 2005
Location: Vero Beach, FL
Posts: 262
70 mph in jeep... 55 stay alive
__________________
'06 Unlimited 4'' OME plus 1" RE spacers...

Once you go black, you dont go back
lostatsea346 is offline   Reply With Quote
Unread 07-11-2010, 07:35 AM   #4
robncar
We Got The Cup, AGAIN!!!
 
robncar's Avatar
2005 TJ Wrangler 
 
Join Date: Nov 2005
Location: La Verne, CA
Posts: 8,359
Quote:
Originally Posted by VnutZ View Post
Target 2700-2800. That's where I was sit on 35 (4.56) and it runs great. I've been on the full gamut of tire/gear combinations and this has proven the most ideal for me as a highway DD.
You have a manual, the OP has the 42RLE, different animal. It's that 0.69 OD that requires lower gearing (higher numerically).

OP, for 33's with the 6-cylinder and 4-speed auto the recommendation is 4.88.
robncar is offline   Reply With Quote
Unread 07-11-2010, 08:02 AM   #5
Unlimited04
This post/info=my opinion
 
Unlimited04's Avatar
2004 LJ Wrangler 
 
Join Date: Dec 2005
Location: Colorado
Posts: 23,322
go straight to 4.88s. do not pass go, do not collect $200. it will be better, but many say 5.13s would be better yet. 35s needs 5.38s or 5.89s. the deepest your D30 can go is 4.88.

the 42RLE's gearing is horrible, it comes undergeared from the factory. a 5-speed manual, with 3.73s and 35s turns the same highway RPMs as a stock 42RLE/3.73s and 30s.
Unlimited04 is offline   Reply With Quote
Unread 07-11-2010, 08:11 AM   #6
LuckRider
Web Wheeler
 
LuckRider's Avatar
2004 TJ Wrangler 
 
Join Date: Apr 2009
Location: NY - 12550
Posts: 9,321
Like everyone said, just go to 4.88 or get a waggy 44 for the front and drop in something deeper (Since you don't have the rubi lockers you can as high as 5.89, but 5.13 or 5.38 would be better for 33"s). Being undergeared from the factory sucks on anything but a flat or slight downhill highway.
__________________
Thanks to SuperWade2 For the 1 Year Upgrade!
Support Our Troops!
ALL WK2 Owners, Click Here
JFI Member #7: Helping out the Scammed
2004 Rubi: Ready to Kill the Chrome!
Totaled and Gone: Mostly Stock 2005 I6 Patriot Blue TJ: Monstaliner (Write-Up) BESRK Rear Bumper, Hella 500s
Do the Jeep Wave :D 0|||||||0 Take 95% of what I say as a joke/sarcasm!-Thomas
LuckRider is offline   Reply With Quote
Unread 05-26-2011, 06:05 AM   #7
1C41N0
Registered User
2006 LJ Wrangler 
 
Join Date: Jan 2009
Location: South Dakota (military), Nebraska
Posts: 147
*bump* I found a guy to do the regearing and dropped off the cash to order the parts. Tentatively it'll be done around 11 June 11. Here's the final set up: 4.88's, (still have the 33's) with Detroit TrueTracs for front and rear. I did the calculations on 4lo.com and my cruising speed will only hit approx 2400 RPM at 75 (interstate driving...maybe) with that set up.

I was wondering why is 2600-2700 is the best cruising RPM for the 4.0? Most sites, including 4lo.com and Quadratec Knowledge center recommend that 2600-2700 RPM?


Secondly, and finally, why is 2600-2700 RPM sought after for the 4.0 when the factory RPM limit is 3,000 RPM? Any input is appreciated, and I hope this thread helps someone else out too!
1C41N0 is offline   Reply With Quote
Unread 05-26-2011, 06:20 AM   #8
Jason
Registered User
2005 TJ Wrangler 
 
Join Date: May 2009
Location: Eaton Rapids, Michigan
Posts: 6,352
The RPM limit is well above 3K RPMS. Here's the peak hp / torque ratings for the six:

4.0L I-6 242 cu in - 2000-2006
* 190 hp @ 4,600 RPM
* 235 ft/lbs @ 3,200 RPM

The reason people shoot for 2600-2700 (or higher) is that's the spot in the power band that has the best efficiency, particularly with larger tires.
Jason is offline   Reply With Quote
Unread 05-26-2011, 06:23 AM   #9
Imped
Web Wheeler
 
Imped's Avatar
2004 TJ Wrangler 
 
Join Date: Feb 2008
Location: Indianapolis
Posts: 19,256
Rotational mass + brick shape + 4.0 = you need some RPM's to cruise efficiently and to provide the motor with adequate leverage. At 70 mph, the resistance against the shape of the jeep is working hard against you. IMO, the 4.0 needs to be at an absolute minimum of 2400 and preferably in the 2600-2800 range at 70 mph. Lower is not better in this case as you've seen with the 42rle lugger. The 4.0 reaches the point of peak volumetric efficiency at about 3100 RPM, so running closer to that is more efficient than lugging it. Regardless, you can't expect to get acceptable fuel mileage at 70 mph due to wind resistance, even if optimally geared. After about 60 mph, the dropoff is sharp.

As for factory rpm limit, not sure what you mean by that. If you're saying you never go past 3k, you may want to do that occasionally. Hell, I probably redline mine at least once a day....that's 5k.
__________________
OlllllllO
Float Test Build
IndyORV
Imped is offline   Reply With Quote
Unread 05-26-2011, 06:53 AM   #10
Dryseals
Registered User
2000 TJ Wrangler 
 
Join Date: Apr 2010
Location: Texas
Posts: 628
Quote:
Originally Posted by 1C41N0 View Post
*bump* I found a guy to do the regearing and dropped off the cash to order the parts. Tentatively it'll be done around 11 June 11. Here's the final set up: 4.88's, (still have the 33's) with Detroit TrueTracs for front and rear. I did the calculations on 4lo.com and my cruising speed will only hit approx 2400 RPM at 75 (interstate driving...maybe) with that set up.

I was wondering why is 2600-2700 is the best cruising RPM for the 4.0? Most sites, including 4lo.com and Quadratec Knowledge center recommend that 2600-2700 RPM?


Secondly, and finally, why is 2600-2700 RPM sought after for the 4.0 when the factory RPM limit is 3,000 RPM? Any input is appreciated, and I hope this thread helps someone else out too!
Volumetric efficiency. An engine is nothing more than an air pump. Due to things like valve timing, valve configuration, port size and length, etc, etc. the engine will pump the charge in and out at the most efficient rate, at a given RPM. You usually see the VE

Couple that with the amount of power to keep the vehicle moving (gearing) and you produce a likable set up.

Not to be confused with peak HP and torque. If your building a dragster, you want the engine spinning at peak HP at the end of the run. You gear for that. On a torque vehicle, you gear to be driving at peak torque. Cruising you want the vehicle at the bottm end of the peak VE curve.
Dryseals is offline   Reply With Quote
Unread 05-26-2011, 07:16 AM   #11
Unlimited04
This post/info=my opinion
 
Unlimited04's Avatar
2004 LJ Wrangler 
 
Join Date: Dec 2005
Location: Colorado
Posts: 23,322
Quote:
Originally Posted by 1C41N0 View Post
*bump* I found a guy to do the regearing and dropped off the cash to order the parts. Tentatively it'll be done around 11 June 11. Here's the final set up: 4.88's, (still have the 33's) with Detroit TrueTracs for front and rear. I did the calculations on 4lo.com and my cruising speed will only hit approx 2400 RPM at 75 (interstate driving...maybe) with that set up.
thats the setup I have now and I like it. still sucks in the mountains, but not near as bad as 3.73s/31s.

Quote:
Originally Posted by 1C41N0 View Post
I was wondering why is 2600-2700 is the best cruising RPM for the 4.0? Most sites, including 4lo.com and Quadratec Knowledge center recommend that 2600-2700 RPM?

Secondly, and finally, why is 2600-2700 RPM sought after for the 4.0 when the factory RPM limit is 3,000 RPM? Any input is appreciated, and I hope this thread helps someone else out too!
This might help:
http://www.jeepforum.com/forum/f9/33...6/#post9838564
Unlimited04 is offline   Reply With Quote
Unread 05-26-2011, 07:40 AM   #12
TheBoogieman
Demonic Jeep Freak!
 
TheBoogieman's Avatar
1998 TJ Wrangler 
 
Join Date: Aug 2006
Location: Charlottesville, Va
Posts: 27,711
Cut the OD off and rpms are fine.
TheBoogieman is offline   Reply With Quote
Unread 05-26-2011, 10:20 PM   #13
jbjeepfever
Registered User
2004 LJ Wrangler 
 
Join Date: May 2011
Location: Boone, Iowa
Posts: 427
Quote:
Originally Posted by Unlimited04 View Post

thats the setup I have now and I like it.

]
X2

I am not in the mountains, this setup runs like a champ and so far seemed to improve my MPG around town and with camper in tow
jbjeepfever is offline   Reply With Quote
Unread 05-30-2011, 05:57 PM   #14
1C41N0
Registered User
2006 LJ Wrangler 
 
Join Date: Jan 2009
Location: South Dakota (military), Nebraska
Posts: 147
Thanks for the background info. I"ll update this post after I get it done in approx 2 weeks.
1C41N0 is offline   Reply With Quote
Unread 06-11-2011, 08:38 AM   #15
1C41N0
Registered User
2006 LJ Wrangler 
 
Join Date: Jan 2009
Location: South Dakota (military), Nebraska
Posts: 147
Dropped the Jeep off at the regear shop, should be done Sunday late afternoon early evening. I'll post results!
1C41N0 is offline   Reply With Quote
Reply

Tags
rpm , cruise , cruising , ideal , 4.0 , lj , wrangler , 3.73 , 33
Thread Tools


Suggested Threads





Jeep, Wrangler, Cherokee, Grand Cherokee, and other models are copyrighted and trademarked to Jeep/Chrysler Corporation. JeepForum.com is not in any way associated with Jeep or the Chrysler Corp.