Lawn Tractor Transmissions: Tuff Torq K46 to K72
Lawn tractors steer with a wheel and drive through a single rear transaxle, so the transmission is one component rather than two, and it is the one that separates a $3,000 tractor from a $6,000 one more than the engine does. Most are hydrostatic units from Tuff Torq, named with a K and a number. John Deere publishes them: the X350 carries a K46, the X570 a K72, and the published top speeds are 5.5 and 7.2 mph. Husqvarna describes the TS 375XD's as a commercial-grade hydrostat with a locking differential. This page explains what the numbers stand for.
How a tractor hydrostatic works
The engine drives a hydraulic pump inside the transaxle; the pedal or lever tilts the pump's swash plate, which sets how much oil reaches the motor and so the ground speed. There is no clutch and no gear change. The differential and axles are inside the same housing. The size of the pump and motor, the axle diameter and the gear set after the motor are what differ between models, and they set how much torque reaches the wheels and how fast the tractor can travel.
The K-number ladder
Tuff Torq's numbers rise with capacity. The K46 is the light-duty residential unit found on most lawn tractors from John Deere, Cub Cadet, Husqvarna and Troy-Bilt. It is rated for mowing and light towing on level ground, and it is usually a sealed unit without a drain, so its oil is not changed in normal service. The K57 and K58 are the next step, with a larger pump and stronger gears, and the K58 adds a drain and a filter on some fittings. The K62, K66 and K72 are the heavy-duty units: larger axles, cast housings, serviceable oil and, on the K66 and K72, an available locking differential. They are what the garden-tractor end of a range carries, and they are rated for ground-engaging attachments, heavier trailers and slopes.
Why it matters more than horsepower
A 22 hp and a 24 hp engine on the same tractor are hard to tell apart. A K46 and a K72 are not. The heavy unit holds speed on a slope without the oil overheating, pulls a loaded cart from a stop, and tolerates a snow blade or a ground-engaging tiller. The light unit does none of those for long. Owners' reports of K46 units losing drive on steep or heavily loaded properties are common enough that makers have moved their higher models to the K58 and above, and it is the reason John Deere lists the transaxle model on every X-series page.
Locking differential
A differential lets the two rear wheels turn at different speeds in a corner. On wet grass or a side slope it also lets the lighter wheel spin while the other stops. A locking differential, offered on the K66 and K72 and on the Husqvarna TS 375XD's unit, ties the two axles together on demand so both wheels drive. It is a feature of the heavy transaxles because the light ones are not built to carry the torque it concentrates on one axle.
Reading a spec sheet
John Deere and a few others name the model. Most makers write 'hydrostatic' or 'pedal-operated hydrostatic' and nothing more, and some residential tractors use a belt CVT or a gear box instead. Where the model is unnamed, published top speed and towing capacity are the indicators: a 5.5 mph tractor rated to tow a small cart is on a K46-class unit, and a 7 mph tractor rated for ground-engaging attachments is not.
What the catalogue shows
The table below groups every lawn tractor whose maker names the transmission, with the published top speeds. The K-number rows are mostly John Deere and Cub Cadet, because they publish the model; the 'model not stated' row is everyone who does not.
Lawn tractors by transaxle, with the speed each one reaches
| Models | Rated power | Top speed | List price (median) | Brands | |
|---|---|---|---|---|---|
| K46 | 15 | 20–24 hp | 5.3–5.5 mph | $3,499 | Cub Cadet, John Deere, Troy-Bilt |
| K57 | 1 | 21.5 hp | 5.8 mph | — | John Deere |
| K58 | 4 | 23 hp | 5.9–6.2 mph | — | John Deere |
| K62 | 1 | 26 hp | 5.5 mph | $4,899 | Cub Cadet |
| K72 | 4 | 24–25.5 hp | 7.2 mph | — | John Deere |
| Hydrostatic, model not stated | 32 | 10.5–25.5 hp | 4–8.5 mph | $2,899 | Craftsman, Cub Cadet, Husqvarna, John Deere, Kubota +1 |
| CVT | 5 | 10.5–15.5 hp | 4.25–5.5 mph | $2,099 | Cub Cadet, Troy-Bilt |
| Gear | 1 | 21.5 hp | 5.7 mph | — | Kubota |
Grouped from the manufacturer's own transmission wording. Most makers name only the type, so the K-number rows are mainly the brands that publish it.
The machines this page was written around
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FAQ
What is the difference between a Tuff Torq K46 and K72?
The K46 is Tuff Torq's light-duty residential hydrostatic transaxle, usually sealed and rated for mowing and light towing on level ground. The K72 is a heavy-duty unit with larger axles, serviceable oil, an available locking differential and a higher top speed. On John Deere's X-series the X350 carries a K46 at 5.5 mph and the X570 a K72 at 7.2 mph.
Can a K46 transaxle be serviced?
Most K46 fittings are sealed with no drain plug, and makers do not schedule an oil change. The heavier K58, K66 and K72 units have drains and published intervals.
Is a locking differential worth having on a lawn tractor?
On level, dry lawns it is rarely used. On side slopes, wet ground or when towing it stops one wheel spinning while the other stalls. It is offered on the heavy transaxles because they can carry the load it imposes.
Why does my lawn tractor lose power on hills?
A light-duty hydrostatic transaxle heats its oil under sustained load, and as the oil thins the drive weakens. It is the most common reported limit of K46-class units on steep or heavily loaded properties, and the reason heavier units exist.
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Independent reference database. Specs compiled from manufacturer sources and normalized for comparison; verify with the manufacturer or dealer before purchase.