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Found a 4.36 Differential but...


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...the advice seems to be to go with a 3.92 if you really want highway speeds.


4.88/4.36 would get me to about 12% better or just over 55 MPH.  4.88/3/92 would get about 25% or mid-60's.


Hate to lose out on a modest increase but also I don't want to do the swap twice.


Any contrary opinions out there?

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I made the swap in my 1964 L-320 about 2 months ago to a 3.70 and I am thrilled with the results. I cruise all of time on the freeway at 70MPH with plenty of power to go.......will accelerate up to 80+ with ease. I must add that this is on flat ground and the SoCal freeways. Now 1st gear is actually usable. I do have a good set of 4.88 gears left over form the swap with about 45,000 miles on them that I will be selling soon. I also have a complete parts truck we will begin disassembling very soon.......sorry no hubcaps.


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I have 390 gears in my NL320, I also have a newer E1(1200) engine, on level ground I can just reach 70mph, maybe a little more, but I cannot hold it on a hill, it likes 60-65mph and has the power to turn them gears barely, but I have also had 437 gears in a 320, that truck had an A15/dual SUs/5 speed in it, and it scooted around real good, I actually liked that truck a lot, it was a mini hot rod, way more friendly for city driving, which is what I mostly do.


Let us face a fact here, how many of us really want to go 80mph in a 320???

If I were to do it again, which I will, I will likely go with 437s, or 411s, these little blocks are made to rev(E1, J13/15, A series), I am fine with anything below 4000rpms, which is 70mph with 437s.

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70MPH on a 23.65" diameter tire with 4.88 is 5K RPMs (I don't know what tires are on the 320 so assuming 6.00 X14?)


70 MPH on same tires with 4.375 is 4,350 RPMs

70 MPH on same tires with 4.110 is 4,090 RPMs

70 MPH on same tires with 3.889 is 3,850 RPMs

70 MPH on same tires with 3.700 is 3,680 RPMs



At some point the motor isn't making enough power to push through the air and any incline or windy days and you won't even get it into 4th.

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Final drive gearing comparisons you should take into account transmission gearing.



For instance I have a close ratio/ tall late model ZX gearbox, 3.062 1st gear and 0.745 5th gear ... so a 3.89 final ratio is a bit tall for my cammed motor, spinning only 3000rpm at 70 and off the line is a bit sluggish.  I'm going to swap a 4.38. 

I find myself cruising at 75-80 to keep the motor in a better power range due to my cam profile.


if you have a super low 1st gear like a 3.6, then yeah a 4.11 or 3.89 would be OK off the line for your smaller engine, but also have to consider power output at cruising speed.

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I run a 4.11 in my z and love it, everyone said it would be too high and to go with a 3.7. 


Yeah ditto !! ^^^ ...  I just put in my 4.11 R180 not long ago... kicks butt even with my L24 (lol)

Keeps me in the optimum power range (as you very well know lol)


I'm curious to try the 4.375 R180 in storage just for the fun of it .. not committed to doing it though... just mostly curious.. lol.


Back on track to the original poster --- you're gonna wanna listen to these guys :)




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