Help figuring out Gear Ratios

[MississippiComanche]

I currently have a 2.5L 4cyl with AX4. I recently upgraded my wheels to 31's, but I still have stock 3.55 gearing. 

 

Since getting larger wheels, I've noticed the sluggishness. What gear ratio should I upgrade to for 31's on my setup? 

[ghetdjc320]

Going to leave this link here for this tool that @derf created a long time ago: http://www.grimmjeeper.com/gears.html

 

 

[MississippiComanche]

Yea I got a little confused by the form. I'm looking for suggestions rather than raw rpm data. 

[Big_Mark]

3:73 will do ya well.

If you plan to go larger than 31's, ever.... go 4:10 but honestly 3:73 is a great ratio all around.

[89 MJ]

I’d run 4.10s. That will be slightly shorter gearing than stock. Cruiser always said that these AMC inline engines were designed to run 24 mph per every 1000 rpms in drive (1:1 ratio). You don’t need to be afraid of turning it some higher rpms. 

[Dzimm]

2 hours ago, ghetdjc320 said:

Going to leave this link here for this tool that @Dzimm created a long time ago: http://www.grimmjeeper.com/gears.html

Just wanted to clarify, I am not the one who created this tool nor do I know who did.  Looks like a great tool though!

[ghetdjc320]

3 minutes ago, Dzimm said:

Just wanted to clarify, I am not the one who created this tool nor do I know who did.  Looks like a great tool though!

My bad I quoted the wrong user. Now I'm trying to remember who it is. The creator of it has the link to that page in his sig. 

 

Edit: found it. Its @derf

[Eagle_SX4]

List of factory gear ratios.

Using the grimmjeeper calculator 4.10 will be closest to stock gearing for you.

[watchamakalit]

Dunno if it helps any but I feel like my 4.10's are a great match to 31's

[MississippiComanche]

Yea I'm looking around at parts to do 4 10 and figure out how much this endeavor is going to cost 

[89 MJ]

It might be cheaper and easier to just swap axles. Any 4cyl, 5 speed XJ or MJ will have 4.10s (most likely). If you could find a D44 or 29 spline Chrysler 8.25, that would be a stronger axle too. 

[derf]

Yeah, that's my calculator.  Glad y'all find it useful.

 

I don't have a "this is optimal" shortcut.  Trouble is, there's so many variables to what is optimal.

 

A high revving motor for mud bogging is completely different than a diesel for rock crawling.  So the ratio you want can vary.  Even in the same truck, the answer depends on too many things to factor in.

 

The RPM data is good if you know where you want to end up.  You can do side by side comparison to match a gear ratio to a tire size change and keep the performance close to stock.  Or if you're always downshifting you can try shorter gears (higher number) to get it where you want it.  If you really scream down the highway in top gear you can see what it takes to make that more reasonable. 

[Smokeyyank]

4.10s, if you're going to pay to put gears in do it once. 

[coheed]

2.8 with an AX5 and 31s on mine and the switch from 3.55 to 4.10s was great. Without the 5 gear anything over 60 would be real buzzy, but I use 4th up until 60 95% of the time.

 

If you decide that swapping axles will be easier then I'd also upgrade to a beefier axle while you're at it. Unless you can find an MJ or XJ that already has both to swap over. 

[Pete M]

2.5L + manual + 31s = I'd do 4.56

[ghetdjc320]

I’d also go 4.56. 

 

[derf]

The AX4 doesn't have overdrive so I'm not sure I'd go to 4.56 with 31s. That puts you up near 4K on the tach on the freeway.  Maybe with an AX5.

[MississippiComanche]

Yea I'm prob doing 4.56. Currently hunting for some axles. 

[Pete M]

20 hours ago, derf said:

The AX4 doesn't have overdrive so I'm not sure I'd go to 4.56 with 31s. That puts you up near 4K on the tach on the freeway.  Maybe with an AX5.

 

I'd be willing to make that sacrifice.     but depends on how much freeway the truck might see.  I regularly drive around with OD off in my PT up to 65mph around town.