Transmission vs differential
The transmission changes the ratio between engine speed and driveshaft speed. The differential, in the drive axle, turns the power 90 degrees, applies a final reduction, and lets the left and right wheels turn at different speeds in a corner.
Why gears are needed
An engine makes useful power over a limited speed range. Low gears multiply torque to start a load moving; high gears let the vehicle cruise without over-revving.
What the differential does
In a turn the outer wheel travels farther than the inner one. Gears inside the differential let each axle shaft turn at its own speed while both receive drive.
Trucks
Heavy trucks often have more forward gears, and tandem drive axles add an inter-axle differential between the two rear axles. The principle is the same; the components and service needs are much larger.
Symptoms and look-alikes
Whine that changes with road speed often comes from the axle; noise that changes with gear selection points toward the transmission. Wheel bearings and tires can sound similar. Symptoms overlap between systems. Confirm with a qualified technician and the manufacturer's service information before replacing parts.
Common questions
- Is a transaxle a transmission or a differential?
- Both — it combines them in one housing, common on front-wheel-drive cars.
Sources & review status
- Motor Service's Automotive Encyclopedia, Jud Purvis and William K. Toboldt, 3rd edition, The Goodheart-Willcox Co., Chicago, c. 1958 (copyright dates 1954, 1956, 1958). PDF pp. 5–6 (contents). Supports: Classification of clutches, transmissions and final drives. Historic source.
- Dyke's Automobile and Gasoline Engine Encyclopedia, A. L. Dyke, 12th edition, second run, A. L. Dyke, St. Louis, Missouri, c. 1920. PDF pp. 9, 29. Supports: Power flow from transmission to axle and the role of the differential. Historic source.
Automatic, CVT and automated manual transmissions in today's vehicles are not described in the source books.
Original Repairius explanation. Not yet reviewed by a qualified technician; not a substitute for the maker's service information.