i suck at math, gearing question@ highway speed

so i'm sitting here bored again which causes me to think about more and more and more projects.

unfortunately one of the top priorities for "some day when i have money" is regearing the axles.

i know i need 4.56 for 33's (cuz i know enough to match them, plus that's what i put on my wifes jeep. lol) but i would like to go with 4.88 or dare i say.......5.13. BUT.

how do i figure out what my RPM at freeway speeds would be??? i'm not so much worried about "what RPM will it do at 80", but more "what is still a comfortable top speed with x.x gears".

i'm figuring that 60-65mph is a crappy but bareable top speed compared to my average 70-80mph.

oh by the way 95XJ, 4.0L , AW4, np31.

http://www.angelfire.com/fl/procrastination/rear.html
http://www.f-body.org/gears/left.htm

5.13's in overdrive at 80 mph with 33's will be around 3000rpm....very comfortable.

Here is another one.
http://www.tyres1.com/calculators.html

At 75 mph (corrected speedo gear-GPS verified) I turn 2800 RPM with 4:88's, 285/75/16's (33x11.50) and an AW4......and I average 18mpg freeway.

2000 4.0 with 103K miles

HTH

if you plan to stick with the D30 front axle stick with 4.56s.. 4.88s are available but they wont be that much of an improvement and it will have a very small pinion.. too small for my liking. whichever gear you chose do not be afraid of letting the 4.0L turn 3000 RPM on the highway. it likes to rev and commonly gets better fuel economy as well.

XJEEPER said:

At 75 mph (corrected speedo gear-GPS verified) I turn 2800 RPM with 4:88's, 285/75/16's (33x11.50) and an AW4......and I average 18mpg freeway.

2000 4.0 with 103K miles

HTH

Click to expand...

WOW......i'm liking those number. thanks.


yeah i'm aware of the little pinion, but i don't think it would be a problem. i would really like the low gearing. and gearing is definitely cheaper than a atlasII

scorpio_vette said:

WOW......i'm liking those number. thanks.


yeah i'm aware of the little pinion, but i don't think it would be a problem. i would really like the low gearing. and gearing is definitely cheaper than a atlasII

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Unless you plan on running 35"+ tires, locking up the front and have a really heavy right foot, the D30 pinion size/strength difference between 4:56 and 4:88 is a non-issue. You'll break either ratio just as easily if you hammer on the D30 hard.

Ran 4:56's with 33's on my last Jeep for 3 years with mixed driving and mileage was more like 15mpg ave, 4:88's are perfect for a 33" tire IMO.

like i said above. i currently have 4.56 on my wifes tj with 33's. but she's got a manual, so there always was already a little "more pep in the step". i don't know if it's because i grew up with manuals, or just from the way it feels, but other than purpose build power applications, i've NEVER felt a auto tranny that seemed to have the pull that a manual tranny had. i like the way the gears feel on my wifes tj, but since i do a tiny little more wheeling (specially alot more aggresive terrain), i would really like low gearing. but as i also mentioned. it's a daily driver, and the nearest wheeling is at least 3-4hours away.

i'm so jealous of people that live withing 1hour of nice wheeling or off road parks. i really would like to move.

I have 4.56's on my 2001XJ with BFG AT' (32x11.5) which actually measure just under 31 and I turn about 2650 RPM to go around 70MPH. I am averaging 15-16MPG and I have the right speedo gear.

JohnX said:

http://www.angelfire.com/fl/procrastination/rear.html
http://www.f-body.org/gears/left.htm

5.13's in overdrive at 80 mph with 33's will be around 3000rpm....very comfortable.

Click to expand...

ehh, I got dead on 3K with 35's (34.7" OD) and 5.13's. 200 rpms can make a large difference IMHO.

Here is another calc. does crawl ratio and you can compare different setups.

http://www.grimmjeeper.com/gears.html