Reducing noise levels with taller gearing???


Bigger Hammer
Jan 29, 2010
Eagle, Idaho
First Name
Willys Model
  1. Wagon
Willys Year:
  1. 1959
I am in the process of finding a different rear differential for my wagon. I was thinking that by reducing RPM, I would reduce the noise. My original differential is a 4.27, would installing a differential in the range of 3.31 significantly reduce the noise level in the vehicle ????
It may but will you still get up the hills? Or will you have to change down a gear and then be as noisy as before?
Have you already fitted a overdrive unit?
I have repowered with a small block chevy and the vehicle will be used mainly on the street. The overdrive units seem expensive ( $1000)
chevy sm 420, 4 speed with creeper first gear, I was going to keep the spicer 18 transfer case in place. I suspect with taller gearing that I would reduce RPM about 20-25%, about the same as an overdrive. Do the guys running an overdrive notice an appreciable reduction in noise? Any thoughts?
I believe I can say with relative comfort that if you are experiencing excessive noise from the rearend, it needs work, or maybe it's from the tires on the road, or maybe the transmission bearings. Maybe the axle bearings are worn, maybe the gears are not meshed properly anymore. This problem requires some troubleshooting prior to a complete gear/axle swap. I have the original 4.88's in my wagon and have no such noise. The gearing itself has nothing to do with an uncomfortable noise level. Something else (maybe in that area) is going on.
Can you cruise at 55MPH with the 4.88 gears and still be able to talk or listen to the radio?? I just remember 25 yrs ago in high school when a friend had a cj5 and it seemed impossible to talk in the vehicle and it maybe due to the fact that bearings may have been worn as you suggest. Thanks for your input.
The problem you'll run into is that if you wagon is a 4x4 model, the Dana 25/27 never came with gears higher than 4.27 I believe. Do you have the original rear axle?

I have a 4 speed top loader in my later CJ, and after replacing the input shaft bearing, I couldn't believe how much quieter it was. If you're in 1:1 4th gear, does it make less noise? Where is this noise coming from? Or do you just mean engine noise and you're trying to reduce RPMs to keep the engine noise down?
I did check the service manual and a 4.27 is listed. I haven't gotten the vehicle on the road yet. My son and I are still building it. What rear end ratio are you running and how is the cab noise?
I was just trying to reduce rpm and thought that this would reduce noise but was not certain so I was trying to get an opinion from the group and was hoping that the overdrive group could give an opinion, since this would be a similar drop in rpm.
Can you cruise at 55MPH with the 4.88 gears and still be able to talk or listen to the radio??
Gary, My answer is yes. If i'm going down the road at 55 / 60 mph I can still hold a conversation without yelling. :) Of course, take in to consideration that it is a wagon (echo chamber) and I never did get around to putting any sound barrier in. If I ever get around to doing that, it'll be twice as quiet and stop some heat as well.

I just remember 25 yrs ago in high school when a friend had a cj5 and it seemed impossible to talk in the vehicle and it maybe due to the fact that bearings may have been worn as you suggest.
In my opinion; Older Jeeps tend to be noisy no matter what.

My suggestion is to go with what you got. You're in the beginning stages of a build, correct? Well, there isn't any better way to learn about your rig than to tear in to it. If you were thinking of a rearend swap, it was going to be pulled anyway, so why not pull it with the intention of getting in there and checking it out. Change the bearings and set the gears. I would just about gaurantee that there is too much play which would of course cause noise. It's cheaper. Plus, you and your son will learn about the set up and be much more satisfied knowing what you did. Have fun and get dirty. :)
The Dana 18 transfer case is a source of noise, the output shaft is gear driven all the time and it causes some gear whine.
Adding some dynamat or underbody coating will help with noise. Adding a small block Chevy will not help with noise. :lol:
great post! I did some substaintial digging myself on this issue during phase one of my build. I decided to stand pat with 4.27 in the 41 rear and 25 back. Good set up for flatlanders and mountain goats. I don't have excessive noise other than the usual winding up so you probably do have some play back there that can be tightened up.

1. I could'nt find new gears (higher than 4.27) for the 41 or the 25 in all of Houston, Texas so they may not be findable at any price. So the gear swap was out.

2. OD is expensive for us weekend rebuilders ($4-500 used, $6-800 new) but from all I've read they are sweet and make a huge difference all around. If I could ever find one reasonable I would jump on it.

3. The other option is a complete axil swap (Paul at Willy's America has some good insight) which requires at least some work making things fit (carriers, drive shafts, ect...). One obvious pro to this option is that newer axils have disk brakes which is another improvement to the ride. Prices on this vary as I did not pursue this one that far.

Good luck, I would recommend you slowly work through some of the obvious culprits listed in previous post and see if you can quiet things down some before you start writing some big checks. It's fun and humbling at the same time.
Thanks for your help. We may just stick with the original diff and see how it works out.