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78 datsun 620 pickup rear end swap


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Looking for info in what rear end ,or third member , i can get to up my top speed abit. My 78 is a 4 speed ,not sure what gear ratio yet,but it winds up about 60-65 .What yr third member can i get that will just bolt up and make a noticeable change in rpms and top speed?

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Engine power and wind resistance are the limiting factors to top speed. A good running 620 will easily do 80+ MPH Your engine is obviously not running at full power. The '78 has a 4.375 ratio and is perfect for high speed.


To go 80 MPH you need 4 TIMES the power you needed to go 40 MPH. Double the speed needs 4 times the power. Going to a 3.90 differential will lower the RPM. Lower RPMs means less power to push the air out of the way.


Stick with what you have and get a tune up.

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I guarantee that if you can't go over 65 with those 4.375s you WILL go slower with anything less. Most likely reason for lack of power....


Fuel filter clogged/ float set too high

Secondary on carb not opening

Choke stuck on

Timing retarded or ignition wires/cap/rotor need replacing

Valves too tight

Head gasket blown

Exhaust pipe crushed or muffler clogged

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I'm not sure that 65 is his top speed, he just said it winds up around then, which makes sense for a 4 speed. I traded Jester his 4.11's for my 3.73's in his 4 speed 620 and said it made a world of difference on the highway.

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Calm down Rick. It isn't running out of revs, it's running out of power. There is something wrong with an L20B that won't go over sixty five. Lowering the RPMs will drop the power even more.


It takes HP to push through the air resistance. Four times as much to go 80 as it does to go 40. If lowering the gear ratio made us all go faster we would put 5 to one over drives in and go 400 MPH. There is a post on Ratsun about attempting to break the 620 land speed record. It's about 130. This is the limit that a modified L20B making an estimated 250??? hp can go.

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My truck is turning 3500rpm at 55mph. I have a tach and I use a gps speedo. 4000rpm gets me 60mph.

I'll double check this vs mikes math. Maybe my clutch is slipping? I'd be thrilled if I was under 4K at 65.


4spd with 4.375


80mph would be tough to listen to, even tougher on mpg.


To answer the OP's question...


Not sure that there is a lower ratio rear end that just bolts in. Maybe from a 720?


Third member spider gear swap by mklotz70



I did the spider gear swap on a '66 roadster diff, it was easy.

I haven't installed it in my truck yet...


You can swap in a 5spd for an overdrive. Lower rpm at cruising speed if you have the power.

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65 mph isn't top speed, it will run 75-80 ,I was only wanting to drop rpms, and make it cruise better at higher speeds and lower rpms,


Best if you say what you mean right off.




Easiest/cheapest is a 4.11 or even a 3.90 differential swap. This would be $25 to $75 for the third member. A five speed would likely be twice this amount.


Typically a mid ratio 5 speed will give you about a 13% over drive 5th gear. A 3.90 will give 12%. (about the same)


A 3.90 will cause your truck to accelerate more slowly in every gear. A proper 5 speed will have no change and the added over drive 5th is the 12% drop in RPMs.



Five speeds...

Fifth gear is intended for cruise speed use only and will drop the RPMs usually about 12-14%. This means that if you normally drive at 3,000 in 4th gear, shifting into an over drive 5th will drop it to 2,640 RPMs. Keep in mind as previously stated that dropping the RPMs lowers your hp level. For example you can't gear a vehicle to go 50 MPH at idle, you have to make enough power to push past the wind resistance. In addition, wind resistance increases 4 times when you only double your speed. At some point you are driving with your foot to the floor and barely keeping up.


Generally a 12% drop in RPMs will gain you a small increase in mileage as trucks are over geared because they have to accelerate, climb hills and go highway speeds while potentially carrying heavy loads. You shouldn't use 5th gear to pass another vehicle, drive into strong winds, climb steep hills or if carrying a heavy load. This is not what they are for and it just puts excess stress on the counter shaft bearings. For this, drop down into 4th which is the only gear that does not use the counter shaft.


Usually a 4 speed will go faster than a 5 speed over drive because the engine is revving higher and making more power.

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65 mph isn't top speed, it will run 75-80 ,I was only wanting to drop rpms, and make it cruise better at higher speeds and lower rpms,

Look for 2wd 720's in a yard near you, my truck had 3.73, a guy I know has 3.89 that's stock in his. I traded my 3.73's for a 4.11 out of a 4 speed 620 and the guy with the 620 said it helped a lot. Just look for a different set of gears and you'll be set.

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The 720 is a gold mine of parts. And the rear gear ratio is even listed on the truck so no guessing. Look on the passenger side inner fender under the hood hings. There is an engine tag that looks like this (earlier ones are aluminum and the lettering embossed but with effort can be read)




On the lower TRANS/AXLE line over on the far right is HF38. The 38 identifies it as a 3.889 ratio. A 41 would be a 4.11, a 37 would be a 3.70 and so on.


This is an H-190 axle. H=  removable third member. 190= diameter of the internal crown gear. Larger is stronger.



Over all the years and options the 720 did come in 3.364, 3.545, 3.70, 3.889, 4.11, 4.375, 4.625. Why the lower gears in the 720 but not in the 620? The later 720 used larger engines with much more torque and at lower RPMs than the little L20B. The lowest ratio 3.364 was used on a special 'Mileage Option' 2wd with a Z20 (2 liter) engine with a special 'wide ratio' transmission with an extra low first gear to allow it to take off from a stop easier. I imagine the mileage was ok but the acceleration sluggish.

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