Should I change my gearing?

Should I change my gearing?
A

atldinan3

Jeeper
Posts
36
Thanks
0
Location
Athens, GA
Vehicle(s)
1984 Jeep CJ-8
So I recently posted a question in the Engine section asking about headers & free-flow exhaust. I posed the same question to my local shop and the owner suggested I skip headers/exhaust and spend my money on re-gearing. He suggested 4.10's. I'm trying to figure out if that is what I should do. Here are my variables:

  1. Jeep will NEVER be off-road (she is my stylish & topless city slicker w/ pickup functionality)
  2. Jeep will spend a lot of time doing 90 min trips back and forth from Ath to Atl. I like to travel ~75 on hwy
  3. Fuel mileage is an important consideration. Prius driving wife will make noise if mpg drops under ~20mpg.
  4. Budget for this project is capped at $1k.
  5. I am running 235/75R15's w/ General Altimax tires (again, for on-road use)
  6. Suspension has been returned to stock specs
  7. I have a stock 5-spd, and I think Dana's in front, rear and xfer case.
  8. PO fully rebuilt AMC 258 i6 / 4.2l w/ Holley 2 barrel

My reasons? I cant maintain a constant speed on even the most gradual incline on the hwy in 5th gear. I certainly can't pass anyone on the hwy in 5th gear. Downshifting to 4th makes it possible, but I recently was beat out by a Prius... I'd like the thing to be at least slightly zippy.

Thoughts?
 
I would agree with the shop.
 
Gears first...then exhaust later, if budget allows. :chug:
 
4.10 seems low...not sure hat you have, but maybe 4.56? Or get a T-176 tranny that will let you zip in 4th. The T-5 has no juice in 5th.
 
Id bet your Scrambler is running 2.72 gears right now. Alot of 80s Cjs used these higher ( numerically lower ) gear ratios to help meet Government fuel efficency standards of the era.

I agree with the shop that you do need to regear , but 4.10 is a pretty big jump and will effect your mileage for sure. Switching transmissions wont help either. Both transmissions mentioned have the same final gear ratio of 1 to 1.

I have to head out to work but heres a good calculator to use.


http://www.grimmjeeper.com/gears.html
 
with only running a 235/75 tire I think 3:55 or 3:73 gears would be better for what you want.
 
atldinan3, My 8 still has the stock 2:73 gears with 30" tires and she does pretty well until steep grades, but she's got Mopar MPI on her too ;). Mine see's a good deal of highway miles and gets 20mpg + loaded with camping gear, so the 2:73's are a keeper for me. I've learned to live with slowing down for a few hills.
 
Old Dog said:
with only running a 235/75 tire I think 3:55 or 3:73 gears would be better for what you want.
Click to expand...


Knowing how small a tire that is ( now :) ) I like the numbers you get with the 3.55 gears. 4.10 is what Id recommend for 32 to 33 inch tires.
 
I agree with the shop too.......... We just did a gear swap on our CJ7 ..... SR4 4spd, Dana 300 tcase.... 3.73 gears inline 6 with a holley 2bbl... this thing screams down the interstate on 35" tires and still has the grunt for off road. With that small of a tire 3.55 should be pleny.... depending on your running gear.... If it is 2.73 you will most likely cross the gear brek and need new carriers. We opted to purchase a complete housing for the Limited slip carrier in the AMC20 , purchased an open carrier for our Dana 30 . The exhaust will definately benifit your mileage but will not cover your surging issue. Hope this helps.
 
You might be having engine issues. With 2.72 gears, at 70 mph the engine should be doing around 2000 rpm. At that low of altitude, the engine should be able to pull it decently. I would check the carb and make sure it isn't the 500cfm version. Then again, the only thing Holleys are good for are wheel chocks.:D 500 cfm is way too big for a AMC 258 i6 / 4.2l and will kill mpg's and bottom end torque. Check the timing also.
 
Thanks for the feedback guys. I am assuming that I have the stock 3.73 and I think I will stick with that. My miles are largely highway, and it seems that there may be more cons than pros to increasing my gearing for such an application.

I do want to do something, so I think I am going to go with DUI, Hedman header (2-in-1), 2.5 pipe, Flowmaster Super 50. I wanted a 70 series, however they dont seem to come any smaller than 2.5".

Also, I am hoping that my engine is not "tired". The PO (a forum member) had an ATK rebuilt engine installed about 18 months ago. Thats a good thing, correct?
 
It couldn't have been as tired as the one I pulled out. 33 years old, bone stock and not very well kept! If you want to trade, let me know! You might want to look at the pics in my gallery first though!


This won't be too expensive Dear! That's what I keep saying. Over and over again.
 
It really depends on what you want bud.

Gearing is a major thing to do, but will yield the most noticable results.

There's a 3 way balance between Fuel economy, Top speed, and accelleration / power.

I have 4.27 gears with 33's. The Jeep will go faster than I need / want to on the highway without problems, the tires are large enough to give it a low enough RPM for me @ 65-70mph still has power when I hit it to get up to 80-90mph (yes tested, no not often! :D ) quickly, and still feels like it has much more to go! (I'm not testing to hit 100mph, at least not anytime soon... too fast for my jeep for now ;)

BUT I know everytime I hit the gas my V8 drains gas! I know cruising at 65-70 uses much more gas than 55mph. It's a concious choice of what i want to do is my point.

Find a nice balance, and then you have options to cruise at 55 to make the "prius driving wifey" (:laugh:) happy and you still have some power for smoking up the tires / getting ahead of the granny during a passing lane!

:chug:
~ Jr
 
So I have discovered that I have a 3.31 Trac-Loc on the back of my scrambler. Would switching to a 3.54 give a noticeable increase in power without making a noticeable decrease in mileage? 3.73?

Is either worth the money? Parts + $75hr shop time
 
My guess is that a shop changing the R&P with new gears and bearings would be around $800- $1000. Thats a guess though as I haven't had any done for a few years.
Is it worth it :dunno: if you had the 2:73's I'd say yes, with 3:31's :confused:
 
Old Dog said:
My guess is that a shop changing the R&P with new gears and bearings would be around $800- $1000. Thats a guess though as I haven't had any done for a few years.
Is it worth it :dunno: if you had the 2:73's I'd say yes, with 3:31's :confused:
Click to expand...

I think that may be on the low end of the price range to regear. Get an estimate on what it'll cost you , then we can determine if its worth it for you. :)
 
With only running 29" tires, I would think that AMC 258 i6 / 4.2l would do good on the highway with 3:31's.
 
At 65 mph, your engine should be doing 2220 rpm. You should be able to cruise along at 70 mph easy. Like I said, you have an issue that has nothing to do with axle gears. Clogged cat? Wrong ignition timing? Tired engine? I would do a compression test before you start thinking about other components.
 
You must log in or register to reply here.

Jeep-CJ Donation Drive

Help support Jeep-CJ.com by making a donation.

Help support Jeep-CJ.com by making a donation.
Goal
$200.00
Earned
$0.00
This donation drive ends in
Donate
Back
Top Bottom