highway speeds between 2000-3000rpm?

[LilRhodyXJ]

im regearing to 4.56s for the future of bigger tires but as of now its gunna be with 31s... i was told by a few people that at highway speeds i'll be between 2000-3000 but that cherokees like this range better... they said mileage is still good and they run smoother that high.. my question is with 31s at 50 auot with 3.55s im at 1800 about how much higher will i be with 4.56s about? how high will i be at 70mph? and can someone explain how running constant 2000-3000rpms is better i thought the lower ur rpm the better mpg ur getting and better it is...
thank u to any one who posts this is extrmly important

[LilRhodyXJ]

anybody?

[5-90]

Ideal "cruise speed" for pretty much any engine is at or near where it makes peak torque - fortunately (for us,) the AMC I6 has a nice flat torque peak.

I quit using fifth gear in my 88, and picked up about 3 mpg in doing so (because I went from cruising somewhere around 2000 rpm to about 2800 - which is right near the torque peak for RENIX, per sim.)

At peak torque, the engine is also running at peak volumetric efficiency (moving the most air per revolution,) which allows it to make the most of the fuel its getting. Also, that allows more torque to spin the wheels to maintain speed - which actually means the engine doesn't have to work as hard as it would at a lower crankshaft speed.

When crunching numbers to select new gears, I tend to take two speeds as benchmarks - 40-45mph and 75mph (expressway and freeway.) That used to be 35mph and 65mph, but speed limits have been allowed to rise slightly... Since I do mainly highway driving, I want the 75mph number to be closer to torque peak than 45mph, but I like to get both close (which requires a little compromise, but it's workable.)

The idea of cruising at as low a crankshaft speed as possible isn't always workable - we've got the aerodynamic profile and efficiency of a brick, and you'll want a little more torque to keep those wheels turning...

5-90

[Dr. Dyno]

im regearing to 4.56s for the future of bigger tires but as of now its gunna be with 31s... i was told by a few people that at highway speeds i'll be between 2000-3000 but that cherokees like this range better... they said mileage is still good and they run smoother that high.. my question is with 31s at 50 auot with 3.55s im at 1800 about how much higher will i be with 4.56s about? how high will i be at 70mph? and can someone explain how running constant 2000-3000rpms is better i thought the lower ur rpm the better mpg ur getting and better it is...
thank u to any one who posts this is extrmly important

The combination of 31's and 3.55 gears yields the following:

Overdrive 4th: 1400rpm at 50mph, 2000rpm at 70mph
3rd gear: 1900rpm at 50mph, 2700rpm at 70mph

If you regear to 4.56's with the same 31" tires, the gearing will be:

Overdrive 4th: 1850rpm at 50mph, 2600rpm at 70mph
3rd gear: 2470rpm at 50mph, 3450rpm at 70mph

You'll be a lot happier if you regear to 4.56's. The Jeep will feel a lot peppier, the tranny will not need to downchange as often on the highway, and gas mileage will improve because the engine will be operating at a more efficient rpm.

[RichP]

True about not using 5th, when I used to commute from pa to nj, 180mi a day on either interstate 78 or interstate 80 and went from the 215's to LT235's I stopped using 5th below 75. Milage went up, if I used 5th below 70 milage went down, even now I don't use it on main hiways even though my commute was much shorter... 82 mi round trip...

[djblade311]

i just regeared to 4.56s from 3.55 on 32s and I can attest to getting my mpg, acceleration, peppiness back. yippee.

You should be fine!

[XJedi23]

What kind of gas milage can I expect from 4.56 w/32" tires. I have a 2000 auto. Im interested in highway fuel efficency due to the distance i must travel.
I may be moving up to 33" tires and would like to get an idea on the correct choice to gain best mpg's

[badron]

==========================================
True about not using 5th, when I used to commute from pa to nj, 180mi a day on either interstate 78 or interstate 80 and went from the 215's to LT235's I stopped using 5th below 75. Milage went up,
=========================================
Sounds like your have a modified or bad MAP, wrong injectors, bad O2(s), custom chip most likely combination of the listed.
Your fuel systems is spending way to much time in the power mode.
There is hundreds of jeeps running around with your tires (I did) and still injoyed the benafit of 5.
================================
The idea of cruising at as low a crankshaft speed as possible isn't always workable - we've got the aerodynamic profile and efficiency of a brick, and you'll want a little more torque to keep those wheels turning...
================================
Same thing the fuel system spends to much time in the power mode. Also systems with EGR systems tend to stay active a lot more if same Jeep is WAY under powered/geared. A bizzy EGR system will send your MPG to the poop can too,
A heavly loaded engine may couse engine knock as the chamber temp rise(EGR working or not) This I turn may forces the backing off of timeing and you MPG heads for the poop can again.

[soarhead25]

True about not using 5th, when I used to commute from pa to nj, 180mi a day on either interstate 78 or interstate 80 and went from the 215's to LT235's I stopped using 5th below 75. Milage went up, if I used 5th below 70 milage went down, even now I don't use it on main hiways even though my commute was much shorter... 82 mi round trip...

I just recently put the 3.5" lift on and went to 31" tires. I haven't even ran a full tank of gas thru it yet to get a baseline on mileage, but for the most part haven't been using 4th gear(D), just been cruising around town in 3rd gear. And with 3rd gear I'm still getting TCC lockup, so I'm happy.
I just hit 4th when on the freeway at 70+
Dealing with a 99xj 3.55 w/auto trans-

Anyone else do this??

[goodburbon]

I just recently put the 3.5" lift on and went to 31" tires. I haven't even ran a full tank of gas thru it yet to get a baseline on mileage, but for the most part haven't been using 4th gear(D), just been cruising around town in 3rd gear. And with 3rd gear I'm still getting TCC lockup, so I'm happy.
I just hit 4th when on the freeway at 70+
Dealing with a 99xj 3.55 w/auto trans-

Anyone else do this??

I drove around on 31's for a year or so and it wasn't too bad, after the gear job it was much nicer though.

[fordtech]

just checked mine today... my speedo is dead on according to my GPS. I ran around 2000 @ 65 in OD. 3.55's, 31's (about 30.6"), dead on corrected speedo.

[53guy]

I've been looking, but can only find a gear calculator for manual tranny's. Does anyone have a calculator that allows for speed, rpm, tire size, gears, and a manual transmission?

[53guy]

Found a really decent gear calculator and made up some stuff for you guys. The first picture is for a Chrysler 8.25 and the second is for a D44. Sorry the pictures are so big, I didn't feel like resizing them, but the detail is good. They are both for RPM's at 75 mph with the AW-4 Tranny and NP 231 TC.



Chrysler 8.25

http://www.fototime.com/{36F97887-706E-4072-8729-96BDAE40D4C5}/picture.JPG

Dana 44

http://www.fototime.com/{C2370378-D3ED-4FD5-A669-DB02D750E872}/picture.JPG

[53guy]

I've been looking, but can only find a gear calculator for manual tranny's. Does anyone have a calculator that allows for speed, rpm, tire size, gears, and a manual transmission?


I meant automatic tranny. Oh well, I found one. :yelclap: