Does Datsun or Nissan have a 0.7×-0.6× 5th gear

[SWFL]

I just bought a 1978 620 with a 5 speed. No idea if it's original yet, as I haven't inspected it. I drove it 1800 miles and for me the 5th gear ratio is far too low. According to my owner's manual, the 5th gear ratio is 0.864, which is not much of an overdrive to 4th gear's 1.000 ratio. I'm fine with a big drop in RPMs and don't want or need a passing/pulling 5th gear, as 4th is just fine for that. I'm happy to swap transmissions to get a nice tall 5th gear ratio in the 0.7 to 0.6 range.

[datzenmike]

Don't forget the overdrive is only for highway cruise and not passing, climbing hills and carrying heavy loads. The 0.864 is 13.6% and reasonable for a heavy truck. There are practical limits to an over drive as they are limited by the engine's ability to make power to pull the truck and push through the air at higher speeds while at the same time at a reduced RPM. That's asking a lot.

 

If this is primarily used for transportation and not work then you could run more over drive but you will notice that you will need to down shift to 4th sooner on grades and while driving into a head wind. 

 

The '81-'83 280zx non turbo 5 speed has an insane 0.745 or 25.5% OD, roughly twice what you have now and is a close ratio box. It was used on a much larger 6 cylinder with lots of torque that could use it. At certain speeds it will lower the RPMs so much the engine won't be able to sustain speed. You will be driving with the throttle more open and engine working harder negating any MPG gain. You simply can't get something for nothing. Not saying it won't work, you just may not like it as much as you expect.

 

The '80 280zx non turbo 5 speed has a 0.773 or 22% OD and is a close ratio box.

 

The '82-'83 S110 200sx has an 18.7% OD but it is a wide ratio box and you will need to put your 4 speed front case on it.

 

The '85-'86 S12 200sx has a 16.7% OD. It's a mid ratio box, you will need to put your 4 speed front case on it, and swap a 56mm counter shaft on.

 

 

Another option to lower highway RPMs is to get rid of the very tall 4.375 differential and replace with something lower. This is much cheaper and easier to do than a replacement transmission.

 

A 4.11 would add 6.4% to your 13.6% for an over all 20% in fifth but it also affects all the lower gears too.

 

A 3.90 would add 12.1% to your fifth for an over all 25.7% but, again, it also affects all the lower gears too.

 

 

 

 

 

 

 

 

 

[SWFL]

Do you know if the 5 speed counter and main shafts have all separate gears or some built in? Some for other vehicles I've seen have 1st and 2nd built in ( VW ) and even 1st-4th built in ( Honda ), with the other gears removable.

 

What has to be done to swap a 280ZX transmission or just the 5th gear? I've never rebuilt RWD boxes, just FWD ones from A-1 chassis VWs and 4th gen Honda Civics.

[datzenmike]

Any L series 5 speed just bolts in place of the 620 5 speed. This is a lot of trouble bother and expense just for lowering your cruise RPMs. You won't go faster, gas mileage increase, if any, will never pay for it and 5th gear will get less use unless you are going fast enough.

 

Fifth gear OD is partly a function of the number of teeth on the gears but also the drive and driven gears at the front. By manipulating the front drive gears the transmission can be made into a wide, mid or close ratio transmission. To get 280zx OD you can't just change the 5th gear. 

 

Below is a mid ratio and a 280zx close ratio box. By swapping the 280zx (close ratio) input gears to the mid ratio box you can  move all the gears closer together but 5th.

 

 

 

 

 

 

[iceman510]

I have an 83 280ZX 5 speed and I would be interested in swapping the 0.745 5th gear OD for your 0.864.  This is something I know has been done before (getting the 0.864  from the 77-79 generation 5 speeds).  As Mike mentioned, the 1980 ZX is 0.773 OD gear.

 

I don't have this thing apart and likely won't have in the immediate  future to do it, but if you are interested, let me know.

 

I also have either a 79 or 80 ZX box as well, but I have never determined which gear set it has.

[datzenmike]

To determine wide, mid or close ratio, set in first gear, turn input 20 times while counting output turns.

 

5.5 turns...... wide... truck

6.0 turns...... mid..... '79 zx, 810 and 620 truck

6.5 turns...... close... only on the '80-'83 non turbo ZX 5 speeds

 

The 5th gear OD is not achieved by simply changing the 5th gear. In fact the mid ratio and the close ratio boxes have the exact same 5th gear sets in them. At the front of the transmission there is a drive gear that spins the countershaft and all the gears on it. By varying these two gears all the other gears are affected including 5th.

 

The 5th gear ratio is 0.591 for  mid and close ratio.

 

The input set for the zx is 23/29 or 1.261 X 5th 0.591 = 0.745 OD

 

The input set for the mid ratio is 22/31 or 1.409 X 5th 0.591 = 0.832 OD

 

If you swap the mid range input gears into a zx transmission you will get the 0.832 OD but you will also change all the other gears too making it a mid ratio.

 

Example... the first gear ratio is 33/14 or 2.357 for both boxes. Multiply this by the input gears and...

 

zx 23/29 or 1.261 X 34/14 or 2.429 =  3.062

mid 22/31 or 1.409 X 33/14 or 2.357 = 3.321

[iceman510]

you are certainly correct it is more complicated then just changing a 5th gear.  I have heard stories of making the combo, but the only guy I know personally who supposedly had done it died a few years ago.

[datzenmike]

I have a much later '85 CA18ET 5 speed in my 710. It's what you would call a wide ratio to get the turbo spooled faster in first. The 5th gear set is actually different than the mid and close. It's .533 so if you put the mid front gears 1.409 in, the OD drops to 0.750. If you put the close ratio front gears in it's OD is 0.672!!!

 

Easier to just get a mid ratio for the 0.864