What Is a Car Differential?
The differential is a gearbox-like assembly that distributes engine power to the driven wheels while allowing them to rotate at different speeds when the car corners.
When you turn a corner, the outer wheel travels a longer arc than the inner wheel. If both wheels were locked to spin at the same rate, the tyres would scrub and skip — and the car would be very difficult to steer. The differential solves this by splitting drive torque between the two wheels, allowing each to turn at its own speed.
Where Is the Differential?
Location depends on the drivetrain layout:
- Front-wheel drive (FWD): The differential is built into the gearbox (transaxle). Most NZ small cars (Corolla, Jazz, Demio) are FWD.
- Rear-wheel drive (RWD): A separate rear differential sits in the centre of the rear axle housing. Common on utes like the Toyota Hilux.
- All-wheel drive / 4WD: Front differential, rear differential, and a centre differential or transfer case.
How Does It Work?
The classic open differential uses a set of bevel gears:
- The ring gear receives rotational power from the driveshaft or input shaft
- Spider gears (small bevel gears on a cross shaft) allow left and right side gears to rotate at different speeds
- The side gears connect to each driven wheel's axle shaft
When driving straight, both side gears rotate at the same speed. When turning, the spider gears allow one side to speed up while the other slows down — automatically, without any driver input.
Types of Differentials
- Open differential: Standard on most passenger cars. Simple and smooth, but sends power to the wheel with least resistance — which causes spinning on slippery surfaces.
- Limited-slip differential (LSD): Uses clutch packs or other mechanisms to partially lock the two output shafts. Common on performance and AWD vehicles.
- Locking differential: Mechanically locks both wheels to spin at the same speed. Used in serious off-road vehicles (Hilux, Land Cruiser).
Signs of Differential Problems
Whining or Howling Noise
A humming, whining, or howling noise from the rear (on RWD vehicles) that changes with vehicle speed — not engine speed — is the classic sign of worn differential bearings or ring-and-pinion gear wear.
Clunking on Acceleration
A clunk when accelerating or decelerating can indicate excessive backlash in worn differential gears or loose pinion bearings.
Vibration
Vibration from the rear of the vehicle, especially at certain speeds.
Leaking Oil
A leak from the differential housing (dark oil on the underside of the rear axle) means the diff seals are worn. Differential oil loss leads to bearing and gear damage. See differential fluid.
Binding or Skipping on LSD/Locking Diffs
If the LSD has worn clutch packs, the differential may chatter in corners or bind when turning tight.
What Happens If You Neglect Differential Maintenance?
Differentials are often overlooked because they're "set and forget" on FWD cars. On RWD and 4WD vehicles, differential fluid must be changed (typically every 40,000–80,000 km or per manufacturer schedule):
- Old, degraded diff oil causes bearing and gear wear
- Low oil from a seal leak can destroy the differential rapidly
- A seized or worn differential can cause complete loss of drive or handling problems
WoF Relevance
A differential oil leak onto the road surface can cause a WoF failure, as it's considered a fluid leak. Additionally, worn differential bearings causing excessive play may be noted by an inspector.
NZ Cost to Repair
| Job | Typical NZ Price |
|---|---|
| Differential fluid service | $100–$200 |
| Differential seal replacement | $200–$500 |
| Differential bearing replacement | $500–$1,200 |
| Rear differential rebuild (worn gears) | $1,200–$3,000+ |
| Rear differential replacement (second-hand) | $800–$2,000 |
For many high-mileage RWD vehicles, sourcing a second-hand differential from a wrecker is cost-effective compared to rebuilding the original.
When to Book a Mechanic
- You hear a whining or howling noise from the rear that increases with road speed
- You notice a clunk when taking up drive from a standstill
- You can see oil leaking from the rear axle area
- You're servicing a high-km RWD vehicle — ask about differential fluid change