33" tires rolling too fast on CJ5

[Hobiedad]

On the 79 CJ5 with the AMC 258 i6 / 4.2l and 3 speed I just purchased, are 33" tires. On the roads and trails this CJ will be on 90% of the time, it just rolls too fast. I'm not sure of the gear ratio, but I assume factory spec. I don't know what that is.

In first gear, I need to be able to crawl up some steep trails and rough roads. Right now it rolls at a fast walk. At highway speeds I'm barely turning 2000rpm.

I may be dropping a tire size because the tires rub the leaf springs on sharp turns. If I'm running 31"-33" tires, is there a preferred ratio to have it creep along in first? I won't be on the highway much. Top speed needs to be 65.

Thanks in advance for your help.

 

[Bill D]

My guess is that you have 3.07 differential gearing. That's what mine was and I had the same problems. Your options are to change the differential gears to something higher (3.73 or 4.11 would be much better) and/or change the transfer gearing to 4:1. I ended up doing both.

 

[HackFabrication]

Too much tire for your gearing (but you knew that already).

With the Dana 20 Transfer Case you can install TeraLow 3.15 gears, which will get you a better crawl, depending on the Transmission in you CJ. 79 CJ5 would have had either the T-150 3spd, or T-18 4sp.

The T-18 has a low first gear: 6.32:1. 2nd gear is: 3.09:1 Similar to the T-150
which has a first gear of: 2.99:1

My 76 CJ5 with Ford T-18 , TeraLow Dana 20 , 4.56 gears, and 32" tires crawls at 1000 rpm in low/low (90.78:1) at 92 feet per minute or 1.05 mph. The trade off is 3112 rpm at 65mph.

Start plugging number into: Gear Ratio Calculator

 

[Hobiedad]

Great info guys. Mine is the 3 speed T-150 . I'll check out Teralow and the calculator to learn more. I'm guessing with the 3 speed I'll be sacrificing more top end than the 4 speed does with something like the 3.15 Teralow.

I appreciate your help.

 

[Hedgehog]

When researching the Terra Low Gears be aware that many of the less expensive gear sets are not for the Jeep application, many are for the Bronco or Scout or HI versions of the Dana 20 . The Jeep gear sets include one more gear so it is more expensive. The least expensive set I found were from ..... Never mind, I can't find the Kit listed at Advanced Adaptors anymore. They were the least expensive at ~$950. Everyone else was ~$100 more expensive. So, for the gear kit alone you will be spending +$1000. Pricey, but for many well worth it. If you don't do it yourself you will be spending +$2000 to gear your differentials.

 

[Posi]

You would typically have 3.54 gears from the factory.
What is your idle set at?
You have a 21.1 crawl ratio your not going anywhere slow with that gearing especially on 33"

Even if you went with 29" tires you still won't have much if any control with your current ratio.

Enjoy yourself offroad and lower your crawl ratio.

 

[Tommy]

I had the same set up as you in my '81 CJ5 . I installed a T-18 to replace the AMC 150 and the difference was dramatic with the granny low. I did not sacrifice my top end speed at all since the 4th gear ratio on the T-18 is the same as the 3rd gear in the AMC 150 . It's a job and it cost me around $850 dollars when it was all said and done. It runs down the highway like it always did and crawls through the rocks just fine.

 

[HackFabrication]

I'm guessing with the 3 speed I'll be sacrificing more top end than the 4 speed does with something like the 3.15 Teralow.

Top end, no. the T-150 and T-18 are both 1:1 in top gear. Your rpm's with your present tire size won't change in top gear. What you are gaining with the T18a is the lower 1st or 'granny' gear at 6.32:1. But, again, that 'granny' 1st gear is not often used on the street.

Going with the TeraLow 3.15:1 conversion will get you lower than the stock 2:1 Dana 20 gearing. But it will only do so much. If you want to go slow, you're going to have to change out the axle gearing.

You would typically have 3.54 gears from the factory. .

That was the most popular gearing back then. My 76 CJ5 had 3.54 gearing before I swapped in the 4.56 gears and ARB's.

 

[Hobiedad]

Thanks everybody for your input. It gave me a lot to consider. I'm thinking I'll do the Teralow 20 and see if that gets me where I need to be. Then I'll look at differential changes if necessary.

 

[GatorBate]

If you drop your Gear Ratio in your differentials to 3.73 or lower you will need to get a different carrier along with the ring and pinion gear. I'm assuming you have the AMC20 in the rear. It has a Carrier split between 3.54 and 3.73.

 

[HackFabrication]

If you drop your Gear Ratio in your differentials to 3.73 or lower you will need to get a different carrier along with the ring and pinion gear.

Yes. All adds to the expense. And ya MAW go with a locker (or two) while you're at it...

 

[Bill D]

Yes. All adds to the expense. And ya MAW go with a locker (or two) while you're at it...

Been there - done that.