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If you are running the 3.9 R200 differential you will want to pair it up with a gearbox that is designed to work with the lower diff gears. The '81 -'83 280ZX non-turbo gearbox was built to work with the 3.9. This match will prevent any abnormal RPM and speedometer problems. Be sure it is a non-turbo, because the turbo based gearbox is meant to be paired with a 3.545 R200 differential. Go to the Transmission Calculator page and play around with the different setups for a detailed RPM to MPH comparison.
These are the numbers for my setup:
- '81-83 280ZX 5-speed
- '81-83 280ZX 3.9 R200
- 6000 redline RPM (I don't know if I would want to rev it any higher)
- 215/45/17 tire size
The chart below shows the difference between both setups given the same 6000 RPM limit and tire size noted above and the gain in mph with the new setup:
| GEAR | 5-Speed & 3.9 R200 | 4-speed & 3.364 R180 | SWAP GAIN |
| 1st | 37 mph | 37 mph | 0 mph |
| 2nd | 61 mph | 59 mph | +2 mph |
| 3rd | 86 mph | 92 mph | -6 mph |
| 4th | 113 mph | 131 mph | -18 mph |
| 5th | 151 mph | N/A | +20 mph |
Since the 5-speed has a closer ratio you see that 3rd and 4th gears actually top out at a lower mph than the 4-speed. But the overall top speed in 5th is much faster. A closer ratio is the way to go for faster acceleration. Another benefit is the RPM at highway cruising speeds. See the chart below for the RPM difference:
| GEAR | 5-Speed & 3.9 R200 RPM @ 80mph |
4-speed & 3.364 R180 RPM @ 80mph |
| 3rd | 5572 rpm | 5218 rpm |
| 4th | 4260 rpm | 3675 rpm |
| 5th | 3174 rpm | N/A |
Let's say to pass another car with the 4-speed I would need to downshift to 3rd. The engine would rev to a numbing 5218 rpm. In the same senario, to pass with the five speed, I would shift down from 5th to 4th to rev the engine at 4260 rpm. This is much lower than redline but high enough to get good acceleration to make the pass, a difference of 958 rpm.
To make the swap all you need is a '81-83 280ZX non-turbo gear box. It is a direct replacement for your 4 speed. I understand that the earlier 5-speeds are not the right length and you need to cut the opening for the shifter. This is NOT the case for the '81-83 5-speed. NO cutting is required.
Just remove the 4-speed and transfer the clutch fork, throwout bearing, and insulator mount to the 5-speed. The mount bracket that bolts the gearbox to the car is a perfect match as well. I used a short-shifter that was made for the 4-speed, so I had to modify the shift mounting tabs with a grinder on the 5-speed. This was the only mod I had to make. Of course you could just use the stock shift linkage that comes with the 5-speed but I didn't have those parts. The short-shifter cost too much not to reuse it and it's much better than the stock setup. Also there is no need to worry about the speedo gear as long as you are pairing it up with a 3.9 R200. The 5-speed uses the white gear where the 4-speed uses the yellow one, in case your wondering. A photo is worth a thousand words, so check them out...
