Off Roading Forums banner
1 - 14 of 14 Posts
G

·
Discussion Starter · #1 ·
I am re-gearing my 85 cj7 (258 auto) for 33" tires and am a little confused on choosing the right gear ratio. I appear to have an AMC 20 rear and Dana 30 Front. We opened the rear and it appears to have 2.73's in it. My confusion stemps from using a Gear calculator (at Randy's ring and pinion) where you put in the ratio, wheelsize and target speed and it tells you the RPM. The numbers I come up with seem a little low on engine speed. So, my questions:

What is good target RPM for highway driving for a 258 auto?
How does one figure the appropriate gearing and can someone point out a good resource for this information?

Thanks for any help,
Justin

 

·
Registered
Joined
·
829 Posts
2500rpm is good on the high way!

Rodney C
1985 CJ7,302 V8,T18,D20,SOA,Shackles Reversed, 35" BFG AT's, Fiberglass Tub & Front End...
1990 Samurai,87 drive line,SOA+2.5" Add-A-Leaf,4.16's,5.12's,33" BFG's Mud's,Pony Carb...
 

·
Registered
Joined
·
2,072 Posts
I have 4.10 gears in my '85 CJ7. I am running 32" tires. My rpms are around 3000-3200 at 70mph. The rpms would be a little less with 33"s. Before I installed my Weber I could barely run at 65mph. My 4th gear is 1:1 which is what your auto should be. I am not sure about the torque converters affect at that speed/rpm.

Later,

http://home.off-road.com/~tom85cj7/home.htm
 
G

·
Discussion Starter · #6 ·
I run my 258 at 1500 to 1800 on the highway at about 55. gets good mileage. 3200 is way to high for the 258.

Since you seem to want to keep your jeep as a dayly driver I would calculate the gears need to return the final drive and engine RPM back to stock. This will return drivability and return your speedometer and odometer to there correct readings. your 235/75R15 where about 29 inches tall. YOur 33'es are about 33 inches tall (mostlikly about 32).

new Gears = new tire size * old gears/old tire size.

For your set up New gears=33*2.73/29 =3.10655

YOu want something close to 3.10 I would go with 3.54's as you can get them used. and they will give you a little better crawing.

bandhmo

 
G

·
Discussion Starter · #7 ·
I had the same combo untill my AMC 20 stripped an axle for the last time. I swapped in a Dana 44 from a Scout with 3.73 gears and 32' tires, the highway RPM is a little higher than stock but it had good acceleration. That is 2741 @ 70 MPH Verus 2200 stock.

Glenn

Build what I can, buy only what I must./wwwthreads_images/icons/crazy.gif
82CJ7, 258, tf999/D300, D30/D44/3.73, SOA, shackle reversal, 32" tires, RS9000s, custom steering / brace, YJ tub, Warn 8274, hi/lift jack, and F/E.
 

·
Registered
Joined
·
2,072 Posts
My Jeep isn't a daily driver. I think that it would be just about perfect with 33"s and the 4.10s. The rpms are a little high with my 32" tires. Since you have an auto you could probably get away with 3.73s. The torque converter helps your crawl ratio a lot. First gear on my tranny is 3.52 so I went with the 4.10s to improve my crawl ratio. I think 3.54s won't be enough, but that is just my opinion. Changing gears is expensive so you only want to do it once. If you think you will ever get bigger tires go with the 4.10s, if you will always run 33"s try the 3.73s. If you are still running the stock 2 piece axles now is a good time to install some Moser 1-piece axles.

Later,

http://home.off-road.com/~tom85cj7/home.htm
 

·
Registered
Joined
·
900 Posts
Justin,

Fill in your profile so we can help....


If you live near California I am going to have my axles (geared 4.10s) out of my jeep soon. I will part with them for cheap or trade for something. The front has a new trac lock. Both were regeared about 20,000 miles ago.

Regearing can be expensive with new gears, install kit, labor.

For what it's worth.



-raymo
4.6 Stroker Scrambler
 
G

·
Discussion Starter · #10 ·
Re: Need ring & pinion advice (Thanks all)

Thanks for the responses. From all I have read it sounds like 3.10s make the most sense as it returns me to my stock RPM at speed after going to 33" tires. I do not do so much crawling these days living in NYC (used to be in Colorado) so daily drivability is paramount (5 hours to Vermont in the winter every weekend). Sounds like going any higher, such as 3.54s, will bring the RPM's up too high at highway speed (65+ MPH).

Thanks again,
Justin
 
G

·
Discussion Starter · #11 ·
Re: Need ring & pinion advice (Thanks all)

3.10 won't do it. You are forgetting something here, the weight on the tires and rims. I have a friend running 3.73's with the 258 and it's good for cruising but not really enough on the trail. Go with the 4.10's, more power off the line and better crawling in the woods. Yeah, go with the Moser 1-piece axles too!
 

·
Registered
Joined
·
650 Posts
Re: Need ring & pinion advice (Thanks all)

I agree. Swapping from 2.73 to 3.10 would be almost silly. Don't forget, the 80's CJ's were so tweaked for fuel economy, everything else was secondary. Lowest I would consider would be something in the range of 3.27 - 3.54.
I did my gears myself, and run 33 BFGAT's. I also have a 5 speed. My Jeep is a daily driver, so I also wanted decent highway performance. I settled on 4.27 and for me it was a good choice.
Bob in Ma
 
G

·
Discussion Starter · #13 ·
My concern with going higher then 3.10s is RPM at highway speed. Here is my thinking

2.73 x 29 x 70 = 2273 RPM (Stock setup)
3.10 x 33 x 70 = 2269 (Bigger tires, basically the same RPM)
3.54 x 33 x 70 = 2591 (Engine is speeding up a little)
3.73 x 33 x 70 = 2730 (Do I really want to run a 258 at this speed for long drives?)

I have had this CJ since 91 and rarely rev it over 3000 or sustain over about 2500. Is my concern of high RPMs unwarranted or this engine?


 

·
Registered
Joined
·
2,072 Posts
The 258 doesn't redline below 4000rpms. My CJ couldn't rev past 3200 before I installed the Weber/HEI combo. Now it will rev all the way to 4000 and it pulls strong. I think you should consider some of Jeeps other factory ratio options that went upto 3.73 and even 4.10 on 4 cyl models.

Here is some axle/ratio info:
http://www.jeeptech.com/axle/amc20.html

Later,

4.10s or at least 3.73s

http://home.off-road.com/~tom85cj7/home.htm
 
1 - 14 of 14 Posts
Top