Here is a question we get more often than you would expect: "I'm building a JZ car on an 8HP, what rear end should I run?"
The honest answer starts somewhere people do not expect. Before you can pick a differential ratio, you need to know which gear ratio set is inside the transmission you bought. ZF did not use one set across the 8HP family, and the difference between them is concentrated exactly where a turbo car cares most: first and second gear.
Two main ratio sets, plus the wide Gen 3 sets
Broadly, three shapes are in circulation among the units swappers actually buy:
| Gear | Set A (early / Gen 1 style) | Set B (later) | Gen 3 wide (e.g. 8HP76) |
|---|---|---|---|
| 1st | 4.714 | 5.000 | 5.500 |
| 2nd | 3.143 | 3.200 | 3.520 |
| 3rd | 2.106 | 2.143 | 2.200 |
| 4th | 1.667 | 1.720 | 1.720 |
| 5th | 1.285 | 1.314 | 1.317 |
| 6th | 1.000 | 1.000 | 1.000 |
| 7th | 0.839 | 0.822 | 0.826 |
| 8th | 0.667 | 0.640 | 0.640 |
| Spread | 7.07:1 | 7.81:1 | 8.59:1 |
Plotted, the useful thing becomes obvious immediately.
Choosing between 8HP ratio sets is almost entirely a decision about how hard the car leaves. It is not a cruising decision, and above third gear it is barely a decision at all.
Why this matters more in a swap than in the donor car
In the car the transmission came out of, the final drive was chosen by the manufacturer to suit that ratio set, that engine and that tyre size. You are throwing all three of those away.
What the difference is actually worth
Hold the differential and tyre constant and compare first-gear multiplication at the wheels:
Worked example
Take a 2JZ car on 26-inch rear tyres with a 3.15 differential.
| Ratio set | 1st × final | 8th × final | Approx. RPM at 70 mph |
|---|---|---|---|
| Set A (4.714 / 0.667) | 14.85 | 2.10 | ~1,900 |
| Set B (5.000 / 0.640) | 15.75 | 2.02 | ~1,825 |
| Gen 3 wide (5.500 / 0.640) | 17.33 | 2.02 | ~1,825 |
Note what that table does not show: a meaningful cruise penalty. Set B and the Gen 3 sets give you more launch multiplication and a slightly taller eighth at the same time. That is the whole point of the wider spread — you are not trading one for the other.
If you already have a Set A transmission and want the launch behaviour of Set B, a 3.31 differential gets you most of the way there. If you have Set B and want to recover cruise RPM, a 2.93 gives you back what the taller first cost you. The ratio set is not a decision you are stuck with — it just changes where your ideal diff lands.
Which set is in your transmission?
This is the part where we have to be straight with you. The ratio set does not map cleanly onto the family number. Two units both stamped 8HP70 can carry different first gears depending on generation and application, and published charts on the internet disagree with each other more often than they agree.
The reliable path is the same one we recommend for every other 8HP question: identify the unit properly from the nameplate and the donor vehicle, then confirm the ratio set against the ZF part number rather than the family. Our ZF 8HP versions guide covers how to read the nameplate, and the used-core checklist covers what to photograph before you buy.
Order the diff after you have the transmission in hand and its ratio set confirmed. We have seen more than one build where a 3.73 was fitted on the assumption of an early ratio set, then paired with a later transmission, and the car ran out of gearing halfway down the straight.
Where the driveshaft comes in
Whatever ratio set you end up with, you are having a driveshaft made. The transmission output flange is the point where that conversation starts, and it varies by the OEM the transmission came from rather than by the family number — which is a whole subject of its own, covered in why two 8HP70s are not the same transmission.
BMW 96mm and 105mm, Dodge Mopar car and truck, and Jaguar output flanges converted to 1310 or 1350 series U-joints. Measure your bolt circle before ordering.
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View the collection →Not sure which ratio set you are working with, or what diff to pair with it? Tell us the transmission you have and the tyre size you are running, and we will talk it through.