Gearing for 40's on UD60's........ 4.88 or 5.38?

[MrJonnycat]

So I'm switching over to UD60's on my 392, and I'm having a hard time deciding whether to go 5.38's or 4.88's on 40's. It's my daily driver, and I do use the freeway from time to time as well, but I'm not sure if the 5.38's will be too much for it. Knowing the transfer case is 2.72:1, and NOT 4:1 like all my other Rubicon's I want a good crawl ratio. I would love to go 5.13's, but that's not offered. Anyone have any real world experience with this?

 

[chevymitchell]

Go with the 5.38.

Follow the math.

 

[rcr1340]

I would do 5.38. Your 8th gear OD is .66. Huge overdrive. It will be great on 40s

 

[GATORB8]

Looks like 4.88s will get you the closest to stock, but 5.38s will get your crawl to 73:1 without too much highway impact. Here's the calcs from grimmjeeper.

Stock:

TC HI	TC LO1	TC LO2	TC HI	TC LO1	TC LO2
Gear	Underdrive Hi	Underdrive Lo
Final Drive Ratio
1	20.50	55.76	-	-	-	-
2	13.12	35.69	-	-	-	-
3	8.77	23.87	-	-	-	-
4	7.05	19.18	-	-	-	-
5	5.37	14.61	-	-	-	-
6	4.10	11.15	-	-	-	-
7	3.36	9.14	-	-	-	-
8	2.62	7.14	-	-	-	-
9	-	-	-	-	-	-
10	-	-	-	-	-	-
R	14.19	38.59	-	-	-	-

The following chart lists the crawl speed you will be going while the engine is at a given RPM. Crawl speed is calculated based on the lowest low range (transfer case and underdrive in low range) available in your rig.

FPM	MPH	FPM	MPH	FPM	MPH
Gear	RPM	RPM	RPM
Crawl speed at given RPM in feet per minute and miles per hour
1	115	1.31	461	5.24	922	10.48
2	180	2.05	720	8.18	1440	16.37
3	269	3.06	1077	12.24	2154	24.48
4	335	3.81	1340	15.23	2680	30.45
5	440	5.00	1759	19.99	3518	39.98
6	576	6.55	2305	26.19	4609	52.38
7	703	7.98	2810	31.94	5621	63.87
8	900	10.23	3601	40.92	7202	81.84
9	-	-	-	-	-	-
10	-	-	-	-	-	-
R	167	1.89	666	7.57	1332	15.14

The following chart lists the road speed you will be going while the engine is at a given RPM. Road speed is calculated based on the transfer case and underdrive being in high range.

Gear	RPM	RPM	RPM
Road speed at given RPM in miles per hour
1	4	14	28
2	6	22	45
3	8	33	67
4	10	41	83
5	14	54	109
6	18	71	142
7	22	87	174
8	28	111	223
9	-	-	-
10	-	-	-
R	5	21	41

The following chart lists the RPMs your engine will be turning while driving at a given speed.

Gear
Road RPMs at given speed in miles per hour
1	2106	7370	14740
2	1348	4717	9434
3	901	3154	6309
4	724	2535	5071
5	552	1931	3862
6	421	1474	2948
7	345	1209	2417
8	270	943	1887
9	-	-	-
10	-	-	-
R	1457	5100	10200

4.88s:

TC HI	TC LO	TC LO2	TC HI	TC LO	TC LO2
Gear	Underdrive Hi	Underdrive Lo
Final Drive Ratio
1	24.40	66.37	-	-	-	-
2	15.62	42.48	-	-	-	-
3	10.44	28.41	-	-	-	-
4	8.39	22.83	-	-	-	-
5	6.39	17.39	-	-	-	-
6	4.88	13.27	-	-	-	-
7	4.00	10.88	-	-	-	-
8	3.12	8.50	-	-	-	-
9	-	-	-	-	-	-
10	-	-	-	-	-	-
R	16.88	45.93	-	-	-	-

The following chart lists the crawl speed you will be going while the engine is at a given RPM. Crawl speed is calculated based on the lowest low range (transfer case and underdrive in low range) available in your rig.

FPM	MPH	FPM	MPH	FPM	MPH
Gear	RPM	RPM	RPM
Crawl speed at given RPM in feet per minute and miles per hour
1	117	1.33	468	5.31	935	10.63
2	183	2.08	731	8.30	1461	16.61
3	273	3.10	1093	12.42	2185	24.83
4	340	3.86	1359	15.45	2719	30.90
5	446	5.07	1785	20.28	3570	40.56
6	585	6.64	2338	26.57	4676	53.14
7	713	8.10	2851	32.40	5703	64.80
8	913	10.38	3653	41.52	7307	83.03
9	-	-	-	-	-	-
10	-	-	-	-	-	-
R	169	1.92	676	7.68	1352	15.36

The following chart lists the road speed you will be going while the engine is at a given RPM. Road speed is calculated based on the transfer case and underdrive being in high range.

Gear	RPM	RPM	RPM
Road speed at given RPM in miles per hour
1	4	14	29
2	6	23	45
3	8	34	68
4	11	42	84
5	14	55	110
6	18	72	145
7	22	88	176
8	28	113	226
9	-	-	-
10	-	-	-
R	5	21	42

The following chart lists the RPMs your engine will be turning while driving at a given speed.

Gear
Road RPMs at given speed in miles per hour
1	2076	7264	14529
2	1328	4649	9298
3	888	3109	6218
4	714	2499	4998
5	544	1903	3807
6	415	1453	2906
7	340	1191	2383
8	266	930	1860
9	-	-	-
10	-	-	-
R	1436	5027	10054

5.38s:

TC HI	TC LO	TC LO2	TC HI	TC LO	TC LO2
Gear	Underdrive Hi	Underdrive Lo
Final Drive Ratio
1	26.90	73.17	-	-	-	-
2	17.22	46.83	-	-	-	-
3	11.51	31.32	-	-	-	-
4	9.25	25.17	-	-	-	-
5	7.05	19.17	-	-	-	-
6	5.38	14.63	-	-	-	-
7	4.41	12.00	-	-	-	-
8	3.44	9.37	-	-	-	-
9	-	-	-	-	-	-
10	-	-	-	-	-	-
R	18.61	50.63	-	-	-	-

The following chart lists the crawl speed you will be going while the engine is at a given RPM. Crawl speed is calculated based on the lowest low range (transfer case and underdrive in low range) available in your rig.

FPM	MPH	FPM	MPH	FPM	MPH
Gear	RPM	RPM	RPM
Crawl speed at given RPM in feet per minute and miles per hour
1	106	1.21	424	4.82	848	9.64
2	166	1.88	663	7.53	1326	15.06
3	248	2.82	991	11.26	1982	22.52
4	308	3.50	1233	14.01	2466	28.02
5	405	4.60	1619	18.40	3238	36.79
6	530	6.03	2121	24.10	4242	48.20
7	647	7.35	2586	29.39	5173	58.78
8	828	9.41	3314	37.66	6628	75.31
9	-	-	-	-	-	-
10	-	-	-	-	-	-
R	153	1.74	613	6.97	1226	13.93

The following chart lists the road speed you will be going while the engine is at a given RPM. Road speed is calculated based on the transfer case and underdrive being in high range.

Gear	RPM	RPM	RPM
Road speed at given RPM in miles per hour
1	3	13	26
2	5	20	41
3	8	31	61
4	10	38	76
5	13	50	100
6	16	66	131
7	20	80	160
8	26	102	205
9	-	-	-
10	-	-	-
R	5	19	38

The following chart lists the RPMs your engine will be turning while driving at a given speed.

Gear
Road RPMs at given speed in miles per hour
1	2288	8009	16017
2	1464	5126	10251
3	979	3428	6855
4	787	2755	5510
5	600	2098	4197
6	458	1602	3203
7	375	1313	2627
8	293	1025	2050
9	-	-	-
10	-	-	-
R	1583	5542	11084

 

[chevymitchell]

These charts do a great job at providing an RPM comparison.

The math I mention should include a mass compensation variable.

4.88's would be closest to stock at stock weight.

Drivability would be more my concern rather than finding a suitable RPM.

Whatever the net weight gain is from stock to modified, add 10% for a loaded, wheeling configuration. (Easily 500 lbs for people and gear.)

EDIT: Just FYI. I ordered an XD60/80 axle set-up from Dynatrac with 5.38's and I will be running a 40", as well.

 

[ozon8r]

Jon, What if you want to go to 42s? If you go 5.38s, you will be ready once you tire (PUN INTENDED) of the 40 shoes. And at the rate you re-shoe your 392, I bet you will be lusting after something bigger than 40s real quick. And tons will be beggin you to go bigger, LOL.

 

[jellis4148]

I'm going to do the UD60's, and 5.38. Granted I have the V6, but my thoughts are when running 40's, and all the extra weight of the axles, beadlocks, and tires the 5.38 will make up for that. I have no math to back up my answer, but it seems logical.

 

[MSparks909]

IMO, 5.38s seems deep to me especially with the Hemi. She will be singing on the highway. Lite Brite is running 5.13s in their Dynatrac XD60/80. What 40s are you going to run? If they’re light you could always do 4.88s and swap in a standard Rubicon 4:1 Transfercase. Gets you a solid crawl ratio and won’t kill your highway mileage

 

[SSWIM]

5:38s

Sam

 

[Outrun]

4.88 because this transmission just ain't the same as a JK.

 

[MSparks909]

Bump. Anyone have any 5.38/40” drivability reports?

 

[MrJonnycat]

I ended up going with Fusion 4 x 4’s K Fusion Elite Series 60’s set up. Eaten E lockers front and rear and 5.13 gears. They are installing 35 spline RCV AXLES up front and upgraded 40 spline axles in the rear. I figure that should be able to handle anything I throw at it even in 2 WD.

 

[Qwest4air]

Does anyone else have fusion axles installed in a jl? I’m in the process of installing some now and have some clearance issues in the up travel on the driver side, the tire rod wants to collide with the Pitman arm off the steering gearbox.

And it’s current set up I’m only gonna have about 2 inches of up travel on a 3 1/2 inch lift.

 

[Qwest4air]

Pics

 

[Qwest4air]

It may not be as bad from an articulation standpoint, but if I ever went fast and hit a dip when the entire axle cycles up together I think it’s gonna be a problem. Has anybody found any workarounds other than going with a flat Pitman arm and notching the crossmember?

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