5.38 to 4.88 or overdrive

Beacher425 · Post by **Beacher425** » Fri Aug 23, 2013 10:52 am

looks like i will have to replace the ring and pinion in the rear diff of a 1955 M38a1. Also wanting to get overdrive to decrease engine RPM at 45mph. Not looking to go any faster. Opinions requested on these options:

1. replacing with 5.38 in rear (about $400) and add overdrive (about $1000)
2. Replace with 4.88 in rear (about $400) and front (about $400)
3. Replace with 4.27 in rear (about $300) and Front (about $300)

Is changing the gear ratio only changing the ring and pinion or are there other parts I'm not yet factoring in?

Thanks.

chuck · Post by **chuck** » Fri Aug 23, 2013 4:02 pm

Go with the stock 5.38 gears plus overdrive. The best combo for an underpowered vehicle to begin with. I added overdrive to my M38 and love it.
Chuck

Post by **wesk** » Fri Aug 23, 2013 8:00 pm

The overdrive has it's advantage in top end after you accelerate through the gears. Getting through the gears with 72 HP is a lot tougher with 4.88's then it is with 5.38's. This is why the factory didn't offer the higher ratios until more HP was available such as the Dauntless V6 and the later in-line six.

Powermwt · Post by **Powermwt** » Sat Aug 24, 2013 12:30 am

The MB/GPW got around well enough using 4.88 with only 54 HP.

I have installed and used a Warn overdrive before and they are great, but if the intent is not to go faster I do not see the point in installing aftermarket parts to solve an issue that does not exist.

That 1955 M38A1 got around great the way it was for a lot of years and you have overlooked a 4th option.

4. replacing with 5.38 in rear (about $400). This is the most economical solution.

Hey, just saying.

Beacher425 · Post by **Beacher425** » Sat Aug 24, 2013 5:02 am

thanks all

I have decided to replace with 5.38 and but off the overdrive maybe around Christmas. maybe i'll get lucky and I can find a used one.

Post by **wesk** » Sat Aug 24, 2013 7:26 am

The MB got along with 4.88's because it had 6.00-6 tires. The pilot model M38's came with 4.88's and 6.00-6 but the Army wanted 7.00-6 which led to the 5.38's.

skyjeep50 · Post by **skyjeep50** » Sat Aug 24, 2013 7:30 am

Your jeep will be quite happy with the revs at around 45 mph. Not much need to change gears if that is all the faster you will be traveling. One other option is to use 7.50X16 tires on the A1. That is what I have and it adds about 6 mph road speed at an indicated 50 mph on the speedo.

Powermwt · Post by **Powermwt** » Sun Aug 25, 2013 1:48 am

Yes Wes, but the A1 has 1/3 more HP... 72 vs. 54.

And the low range in the transfer case is lower, 2.43 vs. 1.97.

I did say I thought is would be more economical to go with the 5.38.

Post by **wesk** » Sun Aug 25, 2013 7:59 am

The transfer low range is of no consequence here. We are discussing highway driving. Both transfer are 1:1.

but the A1 has 1/3 more HP... 72 vs. 54

You must compare apples to apples. 54 is Net HP for the WWII Jeep The WWII Jeep Brake HP was 62.5. The M38A1 Brake HP is 72. The net difference is 11.5.

The M38A1 in stock form cruises the highway easily at 55 MPH. I drove mine at that cruising speed every day to work in SD in the 80's. The time proven method to increased highway speeds with the stock engine and at the same time have no adverse impact on the other operating parameters is to add an overdrive.

Adding 4.88 gears and keeping the stock 7.00-16 tires will have an adverse impact on acceleration rates in both high and lo ranges and in both 2 and 4 wd.