4.88s And 33s

I was playing around with Yukon's rpm calculators, and it seems that I have the optional 4.88s in my Jeep. When I drove it home, it would sit around 27-2800 rpm at 67-ish mph. From what I've read online, the gearboxes in these era CJs all had 1-1 ratios in 3rd gear. Couple that with the OD, which I've read is a 25% reduction, and I get .75 to the axle. Plug those numbers in the calculator, and the 4.88s match up. Are there any other factors that I haven't thought of?

https://ibb.co/6FJQs51

2nd question. Theoretical-like, how well (or poorly) would 33s work with 4.88s, a 225, and an overdrive? According to the calculator, 33s would put me at 2500 rpm @ 67mph, but I don't know what 33s would be like on the low end and for crawling.

 

IMHO Dauntless 225 with T18 and D18 plus overdrive with 4.89 FDR on 7.50 X 16” (31”) tires is the all around “most versatile” drivetrain possible for any of the pre 1972 short engine bay jeeps.

Maximum efficient engine velocity for the standard unmodified Dauntless is 2400 rpm. So adjust your desired hiway cruise speed and drivetrain choices accordingly.

 

Thanks for the responses. I'll be away from home for a while, but I'll be sure to check the actual ratio when I get back. I'm glad to hear that the 4.88s+ work well with 33s. That'll be one thing that I won't need to upgrade when I eventually get bigger tires. At least until I grenade the diffs...

 

I love my 4.88 gears and 33 tires locally and off road but I feel like it's screaming a bit at higher speeds. Your OD is the answer to that.

1:1 is the final drive ratio always without and OD as far as I know.

Moving up to 33" tires will change your speedometer calibration. As I understand it, changing the speedometer gears in the transfer case to the ones for 4.27 gears will put you back to being almost correct with your speedometer. I am assuming that your JEEP came stock with 4.88 gears and is set up with the original and correct speedometer gears now.

Stock tires, a 225 and 4.27 gears are often said to be the perfect combo, or compromise, for general on and off road driving.

My JEEP came with 3.73 and a 225 and was sort of not great off road with 33s. On the brakes a lot and not great in the rocks.

 

Right now the speedo is very wobbly but pretty accurate at the midpoint of the movement. The 4.88s seem to be right from what I can tell. Using 4.27 speedo gears is something I haven't heard of before. I'll have to look into that later. My plan was to get some new gears off of Summit for $70 once I'm OK with the tires that I have, but oem gears would probably be a good bit cheaper.

 

I ran 33s with 3.73 gears for a long time. It was a great setup for the streets and i could cruise at 80 mph all day. Last year i regeared to 4.88 and offroad performance is a lot better in my opinion. On city streets the 4.88 are not that noticeable but at at freeway speeds 65 seem to be pushing it. I do have an OD and that should help with steep hills and freeway speeds but i haven't had time to put if in yet

 

Here's what I've come up with:

What you had: https://ibb.co/pLCQR06

What you have currently: https://ibb.co/gZYSjVv

What you (and I) want: https://ibb.co/ccwZ27G

Does that seem about right for your highest gear? Since you have a sm465 we can't compare offroading ability; unless your reference to the 4.88 swap was before you had the new tranny.

 

Did the '67 Dauntless come with tapered 44 axle? Mine were replaced at some point with a flanged axle. I guess it depends on what you plan on doing with it.

 

I believe they came with tapered axles. I don't know what is currently in there.

How do you mean?

 

They switched to the flanged 44 in mid 70.

 

The very first Renegade I’s built in September of 1969 had the Flanged 30 spline axles with the 4.89 ratio.

 

My understanding is that larger tires are more likely to shear the key on tapered axles. I only know of one situation first hand and it was when the jeep was pulling another out of trouble. I know there are a lot of factors involved here but worth thinking about.

 

I have considered some axle swaps, but I'd like to keep the CJ pretty stock drivetrain wise. My position now is to use the stock axles for a while and see just how much crawling I get myself into. If I start breaking parts in common situations then I'll swap axles.

 

I didn't see this part of your post before. That's something I hadn't considered before. 33s would put me at 2422 rpm @ 65 mph, which is alot better than the 2800 it sits at now. Guess bigger tires really are the answer...

Until I break stuff. Then taller gears are the answer.

 

The 33's may put you in the 225 sweet spot rpm wise but just the potential extra weight of those wheels and tires may adversely affect your usable torque. That said, Old Blue, the 68 I purchased from Fireball last summer (225/4,88's, OD) has skinny 32's on likewise skinny Roxor wheels. It will go down the highway way faster than I'm comfortable driving it, mainly due to the stock Ross box arrangement and the resulting lead time needed for the steering corrections------

But maybe my old age reflexes are just coming up short.

 

Yeah like Duffer says the rpm will be near the sweet spot near 2400 rpm.
But know that the mass of wide 33’s may possibly require more torque than the Dauntless can put out.
If that’s the case then the engine will be lugging at it’s design rpm and cannot run efficient.
I’m a skinny 31” tire guy 7.50x16” so that’s really my limit of experience.

 

Whelp, I'll just have to see how it works for me. I'll report back whenever I end up getting 33s.

 

When I was a teenager I thought I had the coolest Jeep ever.
I put a set of wide / 35” tall monster mudders on my 134 Willy’s.
Turned out could not get them to turn faster than 40 mph on the hiway at wide open throttle.
Well anyway at least I was cool for a couple of weeks.

 

You don't need to go fast to look cool!

I guess I'll have to see how I rate on the naive teenager scale. My justification (read "excuse for stupidity") is that I want to run Rubicon-esque trails.
Maybe I'll have to embrace the teenage rice rocket tuner in me and shlap on some turbos, run 30 psi and 50 degrees of timing, and send a rod to outer space.
On the serious side, I'm banking on the 225 and 4.88s to get me through the trails and the overdrive to get me through the streets. I'll just have to wait and see if my Jeep can pull it off as well as the others.

It's funny how 35s look huge on a CJ but normal on any ol' pickup truck.

 

Yep 35's stock on the wife's twin turbo AWD minivan. It needs a lift, skid plates and a low range transfercase.