Re-gearing a 2.5l..?

[Brokeboi]

Morning all,

 

Its probably been asked but i couldnt find it so please excuse ANOTHER question about an underpowered jeep with too big of tires.

 

I have 33's on my '90 2.5l and assuming it hasn't been re-geared, it should be 4.10 from factory. As one would guess, its a pig. Great off road but driving an hour each way is more tedious than the actual wheeling.

 

Is there a better gear ratio i could swap to and make 5th gear useful, maybe even give this old girl a bit of pep in her arthritic step? I'm starting to get cobwebs around 5th gear.. too low of rpm at 60 (if i can even get there) and in 4th the engine is screaming like a freedom eagle on the 4th of July holding a 6er of Bud.

 

 

[derf]

4.88s would get you back to stock-ish performance.  5.13s would be an upgrade.

 

Go to http://www.grimmjeeper.com/gears.html and put in your info.  AX5, NP231 for the trans and tcase.  Then plug in gear ratios and tire sizes and you can see side by side comparisons.

[Brokeboi]

29 minutes ago, derf said:

4.88s would get you back to stock-ish performance.  5.13s would be an upgrade.

 

Go to http://www.grimmjeeper.com/gears.html and put in your info.  AX5, NP231 for the trans and tcase.  Then plug in gear ratios and tire sizes and you can see side by side comparisons.

 

Maybe this is a dumb question to add, but I'm not real familiar with gearing and the math behind it. Would that affect being in 4lo and the speed/drive ability off road? 

[derf]

The only dumb question is the one you don't ask.

 

The math you use is actually simple.  You just multiply the gears together.  Pick a transmission gear and whether you're in high or low range.  Then multiply those two numbers times the axle ratio.  That gives you the overall ratio you're running.  That website shows you all of those calculations in the chart done for you so you don't have to remember the math.

 

Any time you change the axle ratio, you affect both high and low range.

 

When you go with shorter (higher number) gears, it's like down shifting.  If you regear but you want the same type of performance off road in low range, you're just going to have to upshift the transmission.  Say you liked running in first gear low range.  Changing to 4.88s would have you doing the same stuff in 2nd or 3rd gear instead.  If you are used to running in high gear (4th or 5th) in low range, you may consider putting it in high range in 1st or 2nd gear after the regear.

 

Just about every person I talk to who has regeared said they loved the fact that they got effectively more low range than they had.  Even if they didn't use it all the time.  They liked having it available.  And the only people who didn't like it were people who took their regear too far.

[OldSch88L]

I'm running 31s and regeared to 4.56s, and it's bang on like stock. So off of my head, I'd also say 4.88s for 33" tires.

[Brokeboi]

1 hour ago, OldSch88L said:

I'm running 31s and regeared to 4.56s, and it's bang on like stock. So off of my head, I'd also say 4.88s for 33" tires.

Good to know! You know if the d35  rear and d30 front will fit 4.88s? I've read both online, some say yes, some say no, and I don't know which is right. Buy once cry once haha

[derf]

Both axles can go with 3.07, 3.55, 3.73, 4.10, 4.56, 4.88, and 5.13.

 

The numbers may be off by one like 3.54 or 3.55.  Those still go together.

 

One thing to think about too is to consider swapping in a stronger rear axle.  A Dana 35 is somewhat weak and something like a Dana 44, Ford 8.8 or Chrysler 8.25 would be a good upgrade.

[Pete M]

regearing a 35 is commonly considered a waste of money.  but regearing a stronger axle is an investment.   plus with your lift your stock rear driveshaft is the perfect length for accommodating the new axle (the snout of the common upgrade axles is about an inch longer than the 35).

[OldSch88L]

On 10/7/2020 at 8:38 PM, Brokeboi said:

Good to know! You know if the d35  rear and d30 front will fit 4.88s? I've read both online, some say yes, some say no, and I don't know which is right. Buy once cry once haha

Well I have a Dana 44 rear, but was able to buy "thick" gears, while in the D30 front I had to buy a new carrier because there was only standard thickness available. Note that both my axles came from a 6-cylinder truck with 3.07s, so 4.56s were past the carrier break on both axles. So, with a D35 with 4.10s, I don't know right off the bat if 4.88s will fit, nor do I know the carrier break for its 4.10 carrier. BTW, carrier break is the maximum gearset ratio you can put in before needing a different carrier. As the gear ratio numbers increase, the ring gear gets bigger, but the pinion gets smaller. If the pinion gets smaller, you must get the ring gear closer to it; so either by getting a thicker ring gear to compensate (if available) if you go past the carrier break, or get the proper carrier, which will have a thicker mounting surface that will allow you to get even crazier gears

[Swampy]

I had a 2.5 for a while (before it caught fire) running 35/36's on 5.13 gears with an ax15. It ran great through all the gears except 5th. I could go into 5th but the jeep would struggle. But in comparison to running the 2.5 on 36s and 3.55 gears it was a drastic improvement.