'79 350 CJ -Better highway driving - lower RPMs - advice

Hello all, I need some advice and insight. My CJ is pretty heavily modified. I would like to lower my RPMs when running highway speeds of 65-70 mph. Currently I am about 2800-2900 RPMs at these speeds. And yes my Jeep handles fine at these speeds. Although built for climbing, I will use it as a summer driver and just hits some trails.

I'm looking for insight on how to accomplish this. My first thought was to change rear end gears. If this is your suggestion, what gears would you change to? 383? 350? I'm afraid to change too much with my large tires.

A mechanic thought that we may be able to change gears in my Quadra-Tech transfer case.

Thoughts? Do you have experience in this or is it an educated guess. I look forward to the input.

Here are my specifics:
350 ci
400 turbo trans
Dana 44s
409 gears
35s
American Motors Quadra-Trac part time conversion (Borg Warner 1339) without overdrive

 

Welcome @loudnproud ...not too sure about the mechanic who suggested changing gears in the TC as they are not involved during hwy speeds . Changing axle gears is $$$ so to decrease hwy rpms my suggestion is run larger tires which can be less $$.

 

Changing axle gears would be your best bet since you have an automatic, V8, big tires. the low axle gears you don't really need and you can always get a 4:1 kit for the t-case later on. Although in my opinion driving a CJ at those highway speeds is kinda dangerous anyway.

 

Hey @loudnproud . I am no expert in this topic by any stretch. But just my 3 cents..
I'm running 3.73 gears sitting on 35s. Stock t-case. And im running about 1900 rpm at 70 mph. And getting 19mpg. Now I know CJs and JKs are nothing alike. But the gearing is the same. Correct??
I know @OFFGRID has family things to deal with. But he is our for most expert on the shit.

 

Thanks fellas for the advice.

 

Hopefully someone with the same set up can steer me to the correct gear ratio.

 

Any changes since July?

aggrex is correct as TC ratio changes affect Low Range, but High Range is always 1:1. That leaves the axle ratio and Jim Beam is on the closest track. Without an overdrive you’re stuck with a wailing 350 on the highway. Axle ratio change is the only cure either by higher gears (lower numerically), or bigger tires (same effect).

Without knowing exact ratios available for the D44 offhand, for that 65-70 mph speed, some ballpark numbers are:

3:83 gears will yield about 2650 rpm
3:53 will give 2450
3:23 will give 2250

The downsides are that both axles have to be done, and off-road if in low range will suffer—that’s where the TC ratio change will come into play. Of course, depending on what kind of off-roading you do (trail runs and fishing trips vs. harder-core rock climbing and “hey y’all, watch this” kind of stuff), the 350 most likely has more than enough muscle to not bother with the TC ratio change.

Speedometer drive gearing will have to be addressed too, as any final-drive ratio changes will mess with it.

Hope this helps.

 

Thanks CZJ7.
I haven’t taken any steps yet. I should have enough $ saved up to address it this spring. I’m think a 3:50 ratio will be best for me. I still want a decent hole shot that doesn’t lag too much. I appreciate your insight. Very helpful!

 

Most welcome sir!!

I’ll bet your holeshot now is amazing....quite the sleeper!!!

There is another alternative....swap in an overdrive transmission. Hole shot preserved and highway rpm solved. A 700R4 or 200-4R would do it. Additional mods like cross member mounts, TC compatibility, and driveline length come into play.

Axle gears are certainly cheaper.
$$$ fix anything!

 

Hole shot is okay. My CJ is fairly heavy and those 35s don’t help! I will look into these options. Thanks