Changing Axle Ratios?


Bigger Hammer
Mar 2, 2011
Willys Model
Willys Year:
Hi all. New to the forum, and have a ton of questions about my rebuild. I have a lot of different avenues that I can take for this rebuild, as we all know, but I want to do it as cheap as possible. So what I would like to do is keep everything as original as possible, but upgrade just enough to achieve highway speed. I was thinking that I would like to just change the axle ratios, and put in an overdrive unit. My question though is this: What all do I need to buy in order to change the axle ratios? Do I just need the Ring and Pinion set, or is there more to it than that? And also would it be cheaper and less of a hassle in the long run to just buy a front and rear set of scout II axles? Another question is this: If I want to run say 4.09 gears in the rear, does the front axle ratio need to be the same, or can I leave it stock? Cause in all actuality the front axle would only be used for slow off road speed anyway, but I don't know if that puts undue stress on the trans/t-case running two different ratios. As much detail as you guys can give me will be appreciated. This is my first ever rebuild. I'm not a mechanic by any means, but I am mechanically inclined.

Thanks to all in advance.
Yes the front and rears have to match, and that is where the problems start since ring and pinion choices for the dana 25 and 27 are limited.
Dana 25's lowest ratio was 4.27, but dana 27 has 3.54, 3.73, 4.10, 4.27, 4.88, and of course 5.38

What size of tire do you plan on running?
Eman said:
Dana 25's lowest ratio was 4.27, but dana 27 has 3.54, 3.73, 4.10, 4.27, 4.88, and of course 5.38

What size of tire do you plan on running?

It has 32" tires on it right now. Planning on 35" at the biggest.
Axles have to match or you will break things very quickly. Depending on what you are considering as hiway speeds you may reach them with just an overdrive unit. If you have a good lead on a matched set of Scout axles that may be a good alternative. Changing the ring and pinion in a set of axles is not real hard, but if not done properly they will fail quickly so it may be best left to a professional.
If, by "highway speeds" you mean 55 mph (max), you might get away with an overdrive, new shocks and oem brakes (I was never able to achieve this mythical state) in perfect condition. If you want to cruise at 70 mph, you need updated brakes and no doubt steering as well. Don't forget that these vehicles were never designed for high speeds and so are a bit tipsy. Once you move the center of gravity up with big tires, she'll get 'floaty' and much more dangerous.

Please forgive an old man talking about caution...

Yes a Dana 25 - 27 will support a 35" tire, for how long??? My advise would be to change them out to Dana 44's with matching gear ratio's. Randys Ring & Pinion web site has a calculator to tell you what the ration will do for engine rpm.
Safety should be your concern, not just the look you want to achieve and I agree what Pavel6 stated....
I would look at getting some Dana 44's out of a Wagoneer. Make sure that it is the right year, you don't want vacuum actuated hubs, and you want a 30 spline shafts. I am not sure if it will clear in the back but the front will be perfect. for my truck I got Wide Track Dana 44 from a Cherokee Cheif 78 I believe. Not easy to find! But it has the off set diff in the rear which matches my transfercase. :thumbup:

I am running 5.38 with 35" tires, I have an overdrive and can cruise on the high way at 63-65.... I had it up to 70 but that was too much. My truck has had the suspension, breaks and steering completely modernized. For brakes I have a hydroboost setup that gets its boost from the powersteering pump. It makes the truck stop very well.
I was told by the guy at Walcks that if you change both the axle ratios and have overdrive, you won't have enough power to get up to your max speed anyway.
maps_end said:
Here's a link to a speed caluclator.

Not sure how accurate it is, i'm pretty sure it's mostly theory that doesn't take wind resistance and friction into account at all but it sitll might be a pretty good place to start.

I used that speed calculator to help me decide what I wanted. I would say that it is accurate on what speed you will achieve, but the RPM's at which you achieve it are not correct.

If you put in the calculator..... 35 in. Tires, 3rd Gear, Overdrive at 2800 RPM, it says 72 mph. I can get my truck up to 72 mph, but at more like 3200 RPM... My tach bounces so it's hard to tell exactly...