Buyer’s guide
Why a GDB needs a 3.545 rear diff
This is the most expensive mistake available on a Subaru driveline swap, and the reason it keeps happening is that it does not announce itself. Fit a rear differential with the wrong final drive and the car still drives. It reverses, it steers, nothing knocks. What it is doing instead is grinding the centre differential to death from the first metre, and by the time you find out, the part that failed is not the part you fitted.
Written from our own sourcing experience. Last checked 24 September 2026. Related parts: Drivetrain & differentials.
The number you need
A GDB Impreza WRX STI running the six-speed gearbox needs a rear differential of 3.545, which is a 39:11 crownwheel and pinion — 39 teeth on the crownwheel, 11 on the pinion. Divide one by the other and you get 3.545.
The number that gets bought by mistake is 3.900, which is 39:10. Same crownwheel, one fewer tooth on the pinion, and it is a completely different part for this purpose. Both turn up constantly in the same used listings, described in the same words, at the same money.
39:11 is 3.545 and is what you want. 39:10 is 3.900 and is not.
Why 3.545, and not the same number as the front
The instinct is that front and rear should carry the same final drive, and the instinct is right — but the rear differential is not the whole rear ratio.
The gearbox carries the front final drive internally at 3.900. Drive to the rear leaves the same box through a transfer gear that steps down by 1.100. So whatever ratio the rear differential carries gets multiplied by that 1.100 before it reaches the road.
Run the arithmetic on 3.545 and it comes out at 3.8995 — 3.900 for every practical purpose. Front and rear axles then turn at the same speed for the same road speed, which is exactly what a permanent four-wheel-drive system requires.
| Path | Ratio | Effective final drive |
|---|---|---|
| Front (internal to the gearbox) | 3.900 | 3.900 |
| Rear, correct (1.100 transfer × 3.545) | 3.545 | 3.900 — matched |
| Rear, wrong (1.100 transfer × 3.900) | 3.900 | 4.290 — 10% out |
What a 3.9 rear actually does
Fit a 39:10 and the rear axle is asking to turn ten per cent faster than the front, permanently, in a straight line, on a surface with grip. That is not a cornering difference the centre differential is designed to absorb for a moment — it is a speed difference that never goes away.
The centre differential spends its entire life fighting it. On a DCCD car you are doing this to an expensive and not especially replaceable assembly. It will destroy itself, and it will take its time doing it, which is how people end up convinced the diff was faulty rather than mismatched.
A 3.9 rear is not a substitute for a 3.545. The centre diff fights a permanent speed difference and destroys itself.
The gearbox ratios, for completeness
If you are working out overall gearing, these are the internals of the GDB six-speed. The front final drive and the transfer step are the two numbers that decide the rear diff; the gear ratios matter for everything else.
| Gear | Ratio |
|---|---|
| 1st | 3.636 |
| 2nd | 2.375 |
| 3rd | 1.761 |
| 4th | 1.346 |
| 5th | 1.062 |
| 6th | 0.842 |
| Transfer to rear | 1.100 |
| Front final drive | 3.900 |
Reading it off a listing
A good used listing states the ratio outright, either as the tooth count or as the final drive figure. A Japanese listing will often give both, and the second one is the one to trust because it is harder to typo.
- Stated as 39:11, or as ファイナル3.545 — this is the part. Buy on it.
- Stated as 39:10, or 3.9 final — wrong ratio, regardless of how good the rest of the listing looks.
- No ratio stated anywhere — treat as unknown, not as probably right. Ask before bidding, and if nobody will count the teeth, walk away.
- A housing described only as a rigid or solid diff assembly with no ratio and no LSD type is telling you the seller has not looked. Same answer.
What the right unit fits
The R180 housing carrying 39:11 is the same unit across a wide span of cars, which is good news when you are hunting: a diff pulled from a later GRB, GVB or VAB is the part you need for a GDB, and the donor model in the listing title matters far less than the ratio and the housing.
What does need checking is the housing size itself. Subaru used R160, R180 and R200 rear housings across the same era, and a correct ratio in the wrong housing is still the wrong part.
Questions we get asked
- Can I run a 3.9 rear if I change the front to match?
- Not by swapping the rear alone — the front final drive lives inside the gearbox, so matching it means opening the box and changing the internal ratio. At that point you are doing a far bigger job than the one you set out to do, and the 3.545 rear diff you could have bought instead is a bolt-in.
- How do I know it is 39:11 if the seller will not say?
- Count the teeth, or have whoever is handling the part count them. There is no way to tell a 39:11 from a 39:10 by looking at the outside of the housing, which is exactly why so many wrong ones get sold as right ones.
- Does this apply to the five-speed cars?
- No. The arithmetic here is specific to the six-speed, which carries a 3.900 front final and a 1.100 transfer step. Work the same sum for whatever box you actually have rather than assuming the answer carries across.
Why we wrote this down
We source these parts for our own cars, which means working through listings written by people clearing a shelf and deciding what is worth the risk. None of the above is theory — it is the checklist we actually use. No prices here on purpose: they move weekly, and the hammer price was never the real number anyway.
More guides
- Reading Subaru STI six-speed gearbox numbersWhat TY856 codes and 32000A assembly numbers tell you about generation, DCCD and front LSD — and when they contradict the listing.
- Buying used drivetrain parts sight-unseenWhat a listing has to state before it is safe to bid, why clutch-type beats helical on stage, and the Japanese terms that carry the real information.