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Is 5:13 gears to low for a 37in DD?

R

Ron4x4

NAXJA Forum User
NAXJA Member
Location
Reno, NV
I am going to put 5:13 gears in my 14bolt and in the Dana 44, but after talking with a friend. He seems to think I'll be to low, with 37" MTRs as a DD. He recommends 4:88.
You guys have any Ideas?
The plan is for 5:13 with a detroit in the rear, and Aussie locker or a selectable in the front. I am building the Axle brackets right now and plan on hitting Pick and Pull for the 8lug outers and dually hubs for the rear. So my over all axle width wms to wms in the front 63 inches and 63.5 for the rear.
On the corporate, I am putting disc breakes with Caddy calipers
thanks
Ron
 
5.13 gears with true 37" calculates to 80 mph@3000 rpm in OD...

How much faster do ya need to go :dunno:
 
Well right now I run 80+ at 2300, around here if you don't do 75-80 you get run over. Crazy Sac and Bay Area drivers. I thought 5:13s would be fine, but the guy who mentioned it is an "old timer" friend. So I figured I would check with those of you that are running them.
 
35s is 5:13 territory, so 5:38s wouldn't really be too low for 37s. I'm running 4:88s with 33s, and it's really nice on the trail and not at all bad on the freeway.
5:13s would NOT be too low for 37s, and quite possibly would not be low enough for some situations.
 
Ron4x4 said:
I am going to put 5:13 gears in my 14bolt and in the Dana 44, but after talking with a friend. He seems to think I'll be to low, with 37" MTRs as a DD. He recommends 4:88.
You guys have any Ideas?
Click to expand...

Here's an idea. Tell your friend that he doesn't know what he's talking about.
 
ruggedjeep said:
my buddy has 4.88 w/ full detroit front and rear in his yota DD and hes running 37's, he complains he should have gone lower.... so maybe 5:13 aint so bad.
Click to expand...


Yota's are horribly underpowered.

My buddy regeared his 95 runner to 4.88 w/33s, and my Jeep can still smoke it, even when I had the 3.55s.
 
I'd stick with the 5.13's. If you go above 80 on the highway 5.38's would limit it too much, but you need the extra revs to make up for the weight and drag of the tires so 4.88's would be slightly undergeared IMO.
 
BillR said:
35s is 5:13 territory, so 5:38s wouldn't really be too low for 37s. I'm running 4:88s with 33s, and it's really nice on the trail and not at all bad on the freeway.
5:13s would NOT be too low for 37s, and quite possibly would not be low enough for some situations.
Click to expand...

I think 5.13:1 is as low as one can go with a GM 14 bolt FF axle... (but I might have this # mixed up with the HP60?)

80@3000 was where I was at with 4.56:1 and 33" in my 5spd MJ (probably 32" 'true' at road pressure/load) low enough rpm to still have LOTS of throttle left in OD or downshift to 4th and pass.

BTW the simple equation for MPH at a given RPM is:

True tire radius (ground up to the center of the hub) X 2 X R.P.M.
_______________(divided by)______________________
Axle Ratio X Gear ratio X (1 or .whatever overdrive or low range) X 336

This gives you a "theoretical-ideal" speed@RPM... obviously a lifted vehicle with 'big junk' weighs and cuts wind worse than a stocker... so YMMV, maybe alot depending on the health of the engine/trans.

With big axles and tires that allows, I wouldn't go any higher than 5.13...
 
Jes said:
Here's an idea. Tell your friend that he doesn't know what he's talking about.
Click to expand...

Big ditto!!

I run 5:13's and 37" MT/R's. With the new stroker motor the gears are just right, before the stroker I needed an even lower gear.
 
That 's what I thought 5:13 will be fine for a daily driver. Yeah with the 14bolt 5:13 is as low as I can go.
on the rocks I am sure I am going to want more, but for the road 5:13 will be fine thanks for the confirmation guys.
 
Oh man I'm in trouble. I decided on 5.38's with 33's on my '89 auto. I rarely see much over 100 mph so I'm not worried. Do you think I'll blow my motor? :D

ratio_pic.jpg
 
I think an 89 4.0 auto XJ with 33" and 5.38:1 would be sweet. Especially if everything else underneath was built to smack around.
 
woody said:
I think an 89 4.0 auto XJ with 33" and 5.38:1 would be sweet. Especially if everything else underneath was built to smack around.
Click to expand...

I doing it for better compression braking leading up to speed bumps. Those things are killers.
 
Beezil said:
didn't see any one ask "which tranny"
Click to expand...
I haven't posted in this thread, because I don't have the experience to offer an opinion (I'm using 4.88's with 31's on a 5-speed manual tranny - temporary until I trim fenders to install the 35's I have in storage) . . . but, I'm curious why the tranny type makes a difference. I've read this comment before, and it didn't make much sense the first time I read it.

I'm just trying to understand - here's my general thought on gearing . . . go as low as you can (numerically high) for off-highway use, while maintaining an acceptable RPM at highway/freeway speeds (avoiding over-revving). In my opinion, you can't go too low off-highway - even if you're mudding and need high wheel speed, you can always just shift the tranny to a higher gear - so IMHO the real limit is the highway engine speed (RPM's).

I know that the different trannies have different characteristics (including torque multiplication in the auto) that could affect a gearing choice, but since the OD ratio on manual and auto trannies are generally relatively close to each other (I think, generally in the .70 - .79 range), why would it matter which tranny type a person has? They will still yield similar highway RPM's. Am I wrong that the highway RPM's are the limiting factor on gearing? :huh:
 
red87cherokee said:
I haven't posted in this thread, because I don't have the experience to offer an opinion (I'm using 4.88's with 31's on a 5-speed manual tranny - temporary until I trim fenders to install the 35's I have in storage) . . . but, I'm curious why the tranny type makes a difference. I've read this comment before, and it didn't make much sense the first time I read it.

I'm just trying to understand - here's my general thought on gearing . . . go as low as you can (numerically high) for off-highway use, while maintaining an acceptable RPM at highway/freeway speeds (avoiding over-revving). In my opinion, you can't go too low off-highway - even if you're mudding and need high wheel speed, you can always just shift the tranny to a higher gear - so IMHO the real limit is the highway engine speed (RPM's).

I know that the different trannies have different characteristics (including torque multiplication in the auto) that could affect a gearing choice, but since the OD ratio on manual and auto trannies are generally relatively close to each other (I think, generally in the .70 - .79 range), why would it matter which tranny type a person has? They will still yield similar highway RPM's. Am I wrong that the highway RPM's are the limiting factor on gearing? :huh:
Click to expand...
AW4s are .705 or .75, the AX-15 is something like .83 or .85, so it does make a difference.
 
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