The 1966–1977 early Ford Bronco came with either a 3-speed manual transmission (most commonly the Ford Toploader) or a C4 automatic depending on the year and engine combination. The 3-speed manual is preferred for off-road use due to its strength, while the C4 automatic is favored for ease of use in trail and daily driving scenarios.

The standard transmission in most early Broncos was a 3-speed manual. The 3-speed was paired with the Dana 20 transfer case via a twin-stick floor shift. Automatic transmissions, primarily the Ford C4, were available as an option. The C4 is a robust automatic well-suited to the early Bronco's weight and power range.

Yes, overdrive transmission swaps are popular in early Broncos for improved highway fuel economy and cruising comfort. The NV3550 5-speed manual and AOD or 4R70W overdrive automatics are common choices. Each swap requires a custom crossmember, driveshaft modification, and sometimes a transmission tunnel modification. Tom's Offroad carries NV3550 components and swap support hardware. The 6R80 and 10R80 Automatics are also very popular 6 speed and 10 speed automatics that are being used in high-end builds.

The Ford C4 automatic transmission in an early Bronco holds approximately 9–11 quarts of fluid including the torque converter. Use Ford Type F transmission fluid for the C4, do not use Dexron, as the C4's clutch pack friction material is specifically engineered for Type F's higher friction coefficient. Check the dipstick with the engine warm and the transmission in park.

You can identify an early Bronco transmission by the shift pattern (3-speed manual vs. automatic), the external case appearance, and ID tags stamped on the transmission case. Manual transmissions will have a tag on the side of the case; automatics will have a tag near the pan or bellhousing. Also, notating the shape of the oil pan can help identify the type of transmission you may have.