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P0730: incorrect gear ratio — slipping clutches or a lying sensor
P0730 means the gearbox commanded a gear and measured a different ratio. Whether that is mechanical slip or a bad speed signal decides between a fluid service and a rebuild.
P0730 “Incorrect Gear Ratio” is arithmetic, not opinion. The TCM divides input speed by output speed, compares the answer against the ratio it commanded, and complains when they do not match.
That simplicity is what makes the code useful: the measurement is objective. What it does not tell you is why they disagree, and the two possible answers are separated by roughly two thousand pounds.
The two families of cause
The gearbox is not achieving the gear. Clutches or bands are slipping, hydraulic pressure is low, or a solenoid is not delivering the commanded shift. This is mechanical and real.
The measurement is wrong. A speed sensor is under-reading, so the calculated ratio is wrong while the gearbox is doing exactly what it was told. This is electrical and cheap.
Distinguishing them is the entire job.
Telling slip from a bad signal
Slip has a signature you can feel. Engine RPM rises without a corresponding increase in road speed. Under acceleration the tachometer climbs while the speedometer lags. It is worse when hot, worse under load, and worse in the specific gear that is failing.
A bad signal does not. The car drives normally and the code sets anyway, or the ratio error appears in every gear equally, which is a strong hint that a sensor rather than a clutch pack is at fault.
The test: log engine RPM, turbine speed and vehicle speed together, then work through the gears at steady throttle.
| Observation | Interpretation |
|---|---|
| Ratio error in one gear only | That clutch pack or band is slipping |
| Ratio error in every gear | Speed sensor or output sensor |
| Error grows as fluid warms | Mechanical slip — fluid thins, pressure falls |
| Error present from cold, constant | Electrical |
| Turbine speed dropping out | See P0715 |
The “warms up and gets worse” pattern is the most reliable indicator of genuine slip. Hydraulic pressure falls as fluid thins, so a marginal clutch pack that holds cold begins to slip hot.
Check the cheap things properly
Fluid level and condition
Low fluid means low pressure means slip. Burnt-smelling, dark fluid means clutch material has already been shed. Ensure it is checked at the correct temperature and in the correct gear position — many transmissions have a narrow specified window, and a “correct” level measured cold can be significantly wrong. Detail: ATF diagnostics.
Wrong fluid specification
Modern transmissions are highly specific about friction modifiers. The wrong fluid produces slip and shudder in a mechanically sound gearbox. This is a genuinely common cause after a service at a general workshop — see ATF myths.
Tyre size
Fitting non-standard tyres changes the output speed the gearbox measures. A large enough deviation shifts every calculated ratio. Related: wheel and tyre size change electronics.
Battery and voltage
Low supply voltage causes solenoids to respond weakly and pressures to be wrong. Check before anything internal.
What is actually wearing
When it really is mechanical, the usual culprits in order of likelihood:
- Clutch pack friction material worn in the specific gear that fails.
- Seals and pistons leaking so the pack never gets full apply pressure.
- Valve body wear, where bores wear oval and pressure bleeds off internally.
- Pump wear, which lowers pressure everywhere and produces errors in every gear.
- Torque converter clutch slipping, which can present as a ratio error at cruise — see P0740 and P0741.
Valve body wear is worth highlighting because it is often repairable with a rebuilt valve body at a fraction of the cost of a full rebuild.
Why you should stop driving on it
A slipping clutch pack generates heat and sheds friction material into the fluid. That material circulates through the valve body and blocks the small passages that control pressure, which causes more slip, which sheds more material. The failure accelerates itself.
Catching P0730 early sometimes means a fluid service and a valve body. Ignoring it for six months means a rebuild.
Cost expectations
- Fluid and filter service: $120–350.
- Speed sensor: $40–150.
- Valve body, rebuilt: $400–1200.
- Full rebuild: $1500–4000.
- Used gearbox with fitting: $900–2500.
After the repair
Reset transmission adaptations, then drive through every gear cold and hot at varying loads while logging the ratio. Confirm the calculated ratio matches the commanded gear within tolerance at every step, and that no code returns after a full cycle.
Try it on the free tier of the app — pair an ELM327 adapter and log RPM against vehicle speed in each gear. Work out the ratio yourself and compare it against the manufacturer’s figures. Slip shows up in that arithmetic long before it becomes something you can feel.
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