Subaru WRX and STI Transmission Ratios and Gears

What a Gearset Changes

An upgraded set replaces the factory gears with something stronger, or with a spread that suits drag, rally, or road course use. The gear teeth are already meshed and already turning. Moving the shifter does not slam those teeth together. It slides a hub into the gear you selected and sends the power through that pair.

Fifth gear on a five-speed sits outboard of the thrust bearings, in the same pattern as the other pairs. Reverse uses an idler that slides in and spins the pinion shaft the other way. Ring and pinion is the same pair some builders call the crown wheel and pinion.

Helical, Spur, Synchro, and Dog

Factory Subaru gears are helical. The tooth comes into mesh gradually, more than one tooth carries the load, and the gearbox stays about as quiet as stock. The cost is a side thrust the bearings have to absorb. Spur gears, also called straight-cut, use fewer and larger teeth on parallel shafts. They whine the way reverse does. Volume and tone still change from one maker to the next.

Which cut is stronger is not settled. Helical teeth share the load. Spur teeth are larger and do not push sideways. Tooth profile and the steel matter more than a photo of a wider gear. A very narrow race gear can outlast a wide gear with the wrong profile. Choose from the maker’s rating and from how much noise the car can live with. A drag car tends toward spur. A stronger daily driver tends toward helical. Either cut, built for the job, is far stronger than the factory helical set.

A synchro is a cone. It speeds the gear up to the shaft, then the splines lock and the power goes through. A dog is a cog on a slider that the shifter pushes into a receiver on that gear. Race boxes leave more gap in the receiver so the cog can find the slot at high rpm without a full clutch stroke. Flat shifting is a small lift of the clutch or the throttle, just enough to unload the dogs.

A dog box can be helical, spur, or both. People do drive them every day. That takes rev matching, and it is a bad idea on a car a spouse, a friend, or a valet will drive. Any doubt about the technique is a reason to stay with synchros.

Factory WRX and STI Transmission Ratios

A higher number is a shorter gear: more rpm at the same road speed. The 2007-and-later USDM STI uses a taller 2nd, 3rd, and 4th than the 2004–2006 set and keeps the same 5th and 6th. The JDM STI six-speed keeps the early 1st-through-4th gears and uses a shorter 5th and 6th. The 2018 USDM RA is its own row: 3rd is 1.59, and the rest matches the later USDM STI.

The five-speed WRX gets taller in 1st through 3rd for 2008–2014, then the 2015 WRX leaves this list for the six-speed above. A Legacy GT five-speed and a non-RA STI five-speed share one set. The STI RA five-speed uses a shorter 2nd through 5th, and it is the factory set people buy as the low-cost upgrade.

Aftermarket sets come in street, rally, and drag spreads, and some makers will cut a custom. Use that maker’s current sheet. The lists above are the factory rows. Road speed and rpm at a given gear still have to be calculated from the ratio, the final drive, and the tire.

Final Drive and Tire Size

A numerically higher final drive puts more mechanical advantage at the ring and pinion and less stress on the gearset. A smaller tire does the same thing without opening the differential. The stock circumference in this example is 24.9 inches. A common 50-series 16-inch tire is 24.1 inches, and that moves a 3.90 final drive to an effective 4.03.

Most of the five-speeds in the early WRX years use a 3.90 final drive. The 2006–2007 WRX uses 3.70. That taller gear adds about 6% more load. Shift gently, consider a smaller tire, and hold power modifications to what the next gearset will cost.

On the WRX row, 3.54 times the 1.1 center differential lands on the 3.90 front. The STI six-speed is 1.0 in the center, so it needs a 3.90 rear. Those splits were checked against Subaru parts data on November 19, 2004, for the boxes in that retail list. A six-speed swap that ignores them leaves the center differential splitting two different axle speeds.

What Breaks a Subaru Transmission

Shock load is power, times how suddenly it is applied, times how violent that hit is, times everything else about the launch or the shift. It is not a formula you can finish on paper. Manufacturers publish horsepower and torque ratings. Treat them as a general hint. Any set can fail, and the count that matters is how many shock loads it has already taken.

A stock gearbox can live behind a heavily modified engine when the driver learns that transmission’s quirks and shifts to suit them. The clutch is the piece that is supposed to slip and save the engine and the gears. A factory clutch, or an organic disc of similar design, keeps that cushion. A harsher clutch on stock gears moves the shock into the gearset and can bring the failure forward.

The split case is bolted to itself, to the engine block, and to the tail housing. That construction does not twist the gear mesh apart. The gears sit one above the other. A broken tooth is shock load.

That map is a pattern across many cars, and fifth gear has failed too. Other years are scattered. The 2008–2009 five-speeds showed few broken gears then, and the stated reason is the lack of a rear limited-slip. It is a guess tied to those years, not a promise that a later car is immune.

Race teams that stay ahead of it run a new transmission until it fails, write down the starts or the hours, subtract 10%, and replace on that shorter interval. Inspections can sit inside the interval. The point is to change the box before the tooth goes.

The OEM Gearset Path

The lowest-cost step up is a factory set: an STI RA or non-RA five-speed, a Legacy GT set, or another OEM set. As of the March 2002 build date, Subaru moved the five-speed to the wider RA tooth. Transmission code TY754VN2AA is the narrow gear. TY754VN2BA is the wider gear. Many 2002 cars and some 2003 cars still have the narrow code, so a later Subaru set is both new and stronger. Current Subaru gears are the wider tooth.

Two shop processes show up on upgraded gears. Cryogenic treatment is a dry freeze to about −300°F, held 24 to 36 hours, then brought back to room temperature slowly. The part never sits in liquid nitrogen. The aim is wear resistance, and it can also raise toughness and let residual stress out. Shot peening bombards the surface so the layer just underneath stays in compression. Cracks have a hard time starting in that layer. The compressive stress just under the surface is at least half the yield strength of the steel, and treated parts pick up about 20% of life.

The complete-transmission rows on that 2004 list were the same price for the WRX, the RS, the Forester XT, and the Legacy GT. The STI six-speed was the expensive one. Five WRX gears and the main shaft were the line that added up to about $865.

Install, and Matching a 6-Speed

Final assembly is the step that decides whether the set lives. It belongs with a builder who knows these transmissions, has the Subaru factory manuals, and has the Subaru-only alignment tools. A shop without those tools is the wrong shop. Some dog sets need new shift forks or a shifter interlock, and the builder may find more once the case is open. Budget for that before the car comes apart.

Ask the maker these questions before the order:

1. Ratios. Which sets they sell, and whether a custom cut is real.
2. The case. Whether the housing has to be machined for the gears to fit.
3. Warranty. What it covers, and what driving voids it.
4. The rest of the box. Forks, interlock, bearings, and how much extra money to hold back.

Upgraded sets that come back in pieces usually trace to the install, to a recommended part that was skipped (stock shift forks are the common one), or to a driver who has not learned the box.

While the transmission is out, the rest of the driveline is open for a clutch and flywheel, a final-drive change, limited-slip differentials, a stainless clutch line, a short shifter and its bushings, rear differential bushings, subframe lockout bolts, outrigger bushings, and a rear differential. Those are the pieces that share the shock with the new gears.

Five-speed to six-speed

A six-speed swap needs the whole transmission, the six-speed mount, the six-speed clutch fork, and the six-speed knob, linkage, and boot. The driveshaft can come from a six-speed car, from a Subaru that left the factory with an automatic, or from the WRX five-speed shaft after it is shortened.

The rear end is the decision that sets the cost.

• Regear the WRX R160 to 3.90. The rear housing stays. The ring and pinion change so they match the six-speed.
• Change the six-speed center differential from 1.0 to 1.1. The stock WRX R160, with its 3.54 rear, can stay.
• Install the STI R180. That path takes the STI driveshaft, rear half-shafts, rear hubs, and rear brakes. Brembo rears need 17-inch wheels. An older two-pot rear uses the same mounts and fits a 16-inch wheel. A joint setup can keep the R160 outer joint and the original hubs while the inner joint matches the R180.

Converting the car to rear-wheel drive or front-wheel drive moves load off the gearbox and onto whatever driveline remains. Kits for both exist and are rarely used on a car that is supposed to stay all-wheel drive. Rear-wheel drive adds its own stress to the rear parts.

Before the gearbox comes out

1. Learn the shift. A stock set lasts when the driver stops shocking it.
2. Keep an organic clutch until the gears themselves are the upgrade.
3. Read the transmission code on a 2002 or early 2003. Narrow is TY754VN2AA. Wide is TY754VN2BA.
4. Treat a 2006–2007 WRX as the 3.70 final drive, about 6% more load than 3.90.
5. Match the rear before a six-speed goes in the car.
6. Confirm the shop has the factory manuals and the Subaru transmission tools.
7. Hold money back for forks, an interlock, and whatever the open case reveals.

FAQ

Are straight-cut gears stronger than helical gears?

Neither cut wins on paper. Helical teeth share the load and push on the bearings. Spur teeth are larger, do not push sideways, and they whine. Use the maker’s rating and the noise you will accept.

Can I daily drive a dog box?

Yes, if you rev-match and you are the only one who drives it. Any doubt, or any other driver, is a reason to keep synchros.

Do the gears engage when I shift?

The teeth are already meshed. The shifter locks a slider to the gear you picked, and that is the pair that takes the load.

How much power can a stock Subaru transmission take?

There is no horsepower number that predicts the failure. Count the shock loads: how hard the power hits, and how often.

Will a shorter tire help the transmission?

A circumference under 24.9 inches raises the effective final drive and unloads the gears. The 24.1-inch example turns a 3.90 into about 4.03.

What is the cheapest gear upgrade?

A factory set: STI RA or non-RA five-speed, Legacy GT, or another OEM set. A narrow early gearbox also picks up the wider tooth when the gears are current Subaru parts.

What has to match on a six-speed swap?

The rear gear has to agree with the center differential. The car also needs the six-speed mount, clutch fork, shifter parts, and a driveshaft that fits. Brembo rears need 17-inch wheels.

Why do 2006 and 2007 WRX transmissions see more broken gears?

The final drive is 3.70 instead of 3.90, about 6% more load on the gears. Third is the gear that shows up broken on those years.