4.88 Gears Vs 3.73

If you do not really know the correct gearing for you. Would it be better to swap the axles if you could. I have a chance to pick up a complete set of axles out of a 66 V6 4.88 gears open. My front 27 has issues and the 30 I grabbed once upon a time has 4.10 gears. Seems about the same price to change the gears on my rear as it is for the set with 4.88 gears. Bad part is I feel like I will want/need over drive once they are in.

 

4.89 or 5.38 Final Drive Ratio combined with 25% O.D provides best gearing for all Jeep uses (crawling to hiway cruise)
If you don't need your jeep to excel at both you can skimp on either the O.D. or the low FDR.
So you need to think about how you'll realistically use the Jeep.

A cheaper compromise gearing would be something like 4.27 without O.D.
4.27 does not excel on hiway nor for crawling useage.

 

The tires are just rollers. 225/75r15 27 inches I think. The kids are grown. The wife is exed and now it is time for me. 32 or 33 is the plan with 2.5 lift kit.

 

4.88 with an overdrive is what I ordered when I bought my 1969 CJ5 V6 and I was never sorry. Travel for a few hundred miles on the roads was not terrible, and off road was superb. Limited slip in the rear was also an option I got. At that time it seemed nobody was putting lockers in the rear, and almost nobody put limited slip in the front. WCP
I am not sure what size the tires were, I am thinking they were 10x15 I know they were Gates Commando's X On 15 inch rims X 8 inches wide. I have looked on line for a long time to find a set that were still usable, but they stopped making them lots of years ago so not much luck. I found a set of 4 in a barn but the owner said they were starting to show cracking on the side walls. Best overall light Jeep tire I found.

 

I have 32s and 4.88s in my 69. No overdrive. At 65 mph the engine is wound up, but the Dauntless handles it just fine. I put several thousand highway miles on it in the last month, and I have been driving it back and forth to school (about 350 miles each way) for a year now. I would definitely recommend the 4.88s unless you never go off road.

 

Your rear end from your '71 is generally considered stronger than one from a '66.....

 

I would go 4.88s with an overdrive. 4.27s or 4.10s without. IME 4.88s are a bit too deep for highway use without very tall tires. Using the 4.10 30 and new gears in the rear axle would be a fine combination, IMO. You could go lots of places with that combo. Someday you may want to convert to a truck 4-speed to get that ultimate crawler gear. An overdrive with the 4.10s would also have a lot of merit, but IMO you don't need the overdrive with 4.10s.

Unless you have unlimited time and money, any choice of ratios will involve some compromise. If you need to drive the Jeep to the trailhead and back, or you need to drive on the highway every day, and you can't spend a lot more on an overdrive or truck 4-speed, a ratio like 4.10 or 4.27 will work well.

Note that the '66 rear axle with 4.88s is a significant downgrade from your '71 rear axle. The '71 axle should be the much-sought-after offset flanged Dana 44, while the 66 axle will be a tapered nutted 44 (weaker - lower spline count and an axle key to the hub). Also, the '66 axle is an open axle, which is pretty much a cigar butt other than as a replacement part. The 4.88 R&P from the '66 axle will work in the '71 axle, but you will need a new case ... if you went 4.88, you'd take the gears from the '66 axle and put them in your '71 axle with a superior traction device, like a Dana PowerLock or Detroit TrueTrac (or a locker of some kind ... lots of additional discussion possible here).

 

X2 what Timgr said.The D30 is a good improvement over the 27 and as stated the flanged 44 is superior and highly sought after. Install the 30 regear the 44 save money for a granny 4 someday.

 

Another vote for 4.88's with overdrive.
I've put a lot of miles on 2 Jeeps with this setup and it works very well.

 

Or keep the 30, find a centered 44 with 4.10s. Save the money on the overdrive, put in a 20 tc and get rid of the 18 whine. Don't let anyone tell you the little tiny bit lower gear in the 18 is worth the whine either.

 

Funny you should say this. But with headers and 10 inch turbo mufflers I really don't hear much of anything.

 

Yes, it is!

 

It depends on your transmission/tire setup, too. I have the T-98A tranny with 4.27 gears and the jeep does ok on the road, and very well off-road with 31" tires, but I also have the F-head four banger, but with a two-barrel Weber on it. Sorry I have not been on here, but I got my hands on a 1988 Fiero GT in perfect condition, except for a small exhaust leak on the front manifold, which, of course, is the hardest place on the whole car to get to. Pontiac only made 613 of these and fewer than 300 have been accounted for. I haven't had time to drop the engine to get to the exhaust leak , but I am still around and driving the CJ-5. I have almost 1500 miles on it since I got it back on the road. Unfortunately the TO bearing is getting worse, so I need to pull the tranny and replace it. I will be around.

 

4.88 Gears Vs 3.73

4.88 has 39 ring teeth and 8 pinion teeth
3.73 has 41 ring teeth and 11 pinion teeth

The 4.88 pinion is notably smaller and weaker than the much larger 11 tooth pinion.
Depending on vintage and specific axle installed the 30 spline jeep CJ rear axles were 4.89 ratio
4.89 has 44 ring teeth and 9 pinion teeth
4.89 rear gears are stronger than the 4.88 front gear sets.

Optimum gear selection is also dependent upon ones particular engine and the engines RPM ranging ability.
Many engines like the Dauntless develop maximum torque @ near 2400 RPM.

Personally I would choose 3.73 ratio if O.D. unit was not an option.
For instance I would certainly want 3.73 FDR with a Dana 20.
No higher nor any lower (not 3.54 nor 3.92)
3.73 FDR is near ideal for a stock Dauntless jeep engine.

With Dana 18 and 25% O.D. I would certainly want 4.88/4.89.
Again no higher nor any lower (not 4.56 nor 5.38)
4.89 x .75 (25% O.D.) yields a compound FDR of 3.66 which is again ideal for a jeep engine that puts out maximum torque at 2400 RPM

I feel optimum FDR is in large part relative to transfer case selection.
But in totality the choice is about RANGING.
Both RPM RANGING and OVERALL GEAR RANGING plus tire diameter.

 

I know how important it is to get the ratio correct for the type of wheeling you are going to do. I have missed before with the correct ratios. Once on my new Ford 7.3 PS back in the 90's when I ordered it with 3.55 very nice on highway empty. But did not cut it going up grades with 8K in tow. But total amount of cash available for all items means use what you got, to a point. So I am leaning to 4.09 gears in the rear to match my 4.10 in front. Once the T18 is installed and use of 31 or 32 tires all should be good for limited use on our 55-65 mph highway limits here in Oregon and some off road capabilities.

 

Yeah that will give you a 64/1 crawl ratio which should be excellent for most all trail useage.
True rock crawling generally begins around 75/1.
Later on you could add a 3.15 Tera Low gear set for an 82/1 rock crawling ratio.

 

Sorry for jumping in waaaay late and know it’s probably been beaten to death but after reading this all I know it’s individual preferences. But opinions and wisdom is always welcome. So my question is I have a dauntless V6, 33x12.6 tires, headers and 4bbl carb. Currently have 5.38 gears and no OD. I talked to Novak and asked about gears. 90% road 10% trail (dunes, little trails) they said I should go with 4.27 or at most 4.56, and have a chance to do a direct free axle swap for 4.27. Thoughts???

 

Get an overdrive. You won't regret it.