My Next 620 question

[Dawgz83948]

By doing the math I think I have the 4.625 final drive ratio on my 620. Does anyone know of a good source of a 3.89 gear set?

[Z-train]

Your math is wrong,your gearset is 4.37 to 1.The stock motor doesn't have enough HP for a 3.89 gear.

[DatDoug]

Your math is wrong,your gearset is 4.37 to 1.The stock motor doesn't have enough HP for a 3.89 gear.

 

We put a wagon 3.88 into RickRats 73 with stock L16 & it goes just fine :cool:

[Dawgz83948]

Your math is wrong,your gearset is 4.37 to 1.The stock motor doesn't have enough HP for a 3.89 gear.

 

 

So a fresh l20b w/ a webber can't move a 2300lb 620 w/ a 3.89, I think you're wrong...... I'm 99% sure it's a 4.625 because a 4.37 at 3800 rpm's with 195 60 14 tires = 70mph

[datzenmike]

Only the automatic L16/18s in the 620 got the 4.625 ratio. All standard tranny 620 (except '79) got the 4.375 ratio differential.

 

 

So a fresh l20b w/ a webber can't move a 2300lb 620 w/ a 3.89, I think you're wrong...... I'm 99% sure it's a 4.625 because a 4.37 at 3800 rpm's with 195 60 14 tires = 70mph

 

4.375 diff with 195/60R14 tires @ 3,800 RPMs is 69.44MPH assuming a 620 or 280z 5th gear ratio of 0.864. So, I agree with your math. Which doesn't tell us much about what your truck is doing at 3,800 RPMs to make you think you have a 4.625.

 

A 4.625 ratio would have you going just under 65.7MPH @ 3,800

A 3.889 ratio would have you going just over 78 MPH @ 3,800

 

Yes an L20B will pull a 620 with 3.889 gears but it will have sluggish acceleration in all gears. Comparable to a 510 wagon fully loaded. Additionally the engine will be making less RPMs at the same 70 MPH speed. Less RPMs = less power at the same speed so you will have to step down harder on the gas to push through the air.

 

Keep in mind that to double your speed it doesn't take twice the power... it takes 4 Xs as much power. So if you are expecting to go faster at the same 3,800 RPMs because of a lower gear diff you may find the 3,800 RPM power at 70 won't be enough at 78. In fact it's possible that you could go faster in 4th than in 5th. This is called power limited.

[datsunaholic]

Unless the truck was an automatic originally (since it's a '74, the 4.625 ratio was used then) the likelihood of it having the very rare 4.625 ratio is pretty small. With the exception of '73 and '79, all manual 620s were 4.375. More likely you have an unknown speedometer drive cog in there, and the speedo is off. The fact that the trans isn't stock reinforces that.

 

Typically I daily-drive a 4-speed truck on the freeway. Heck, I drove cross-state averaging 75 MPH the last 2 summers and sometimes when I got in a pack it was 80. Yet I constantly hear folks complain about over-revving at 65-70, even with 5-speeds. These aren't big V-8s. They can take the RPMs. 4500 is NOTHING. Just loud.

 

With a L16 or even a stock L20B the 3.88 gears are really too much. Unless you like seeing loaded garbage trucks out-accelerate you.

[Dawgz83948]

The axle appears to be from a junk yard (it has the tell tale paintpen marks on it) so I can't guarantee it's NOT out of an automatic car. Maybe 4.11's would be better.

[yello620]

I have ran 4.37, 4.11, 3.7, and 3.89 gears in a 620. All with the same motor and a 4 speed. The best gear(IMHO) was the 3.89. However, when i swapped in the 5 spd, the 3.89 was a little too much, and i think that the 4.11 would have been better.

 

 

I did not make the change back to the 4.11 gears though. And i am now running a VG30 with the 3.89 gears in the 620, so anything that works for me now really doesnt apply to anybody else.

 

But, with a L20b AND a 5 speed i would recommend the 4.11 gears. An L20b with a 4 speed(and no 5 spd swap plans) i would recommend the 3.89 gears.

[tomyboy]

Snap!! Derailed

[jon521]

im running an L20b with a 5 speed with stock 521 gears which i BELIEVE (not possitive, would have to look up) are 4.11 and they are great :) granted i havent tried any other gears