An automatic three-speed transmission was also offered as a factory option, but was unpopular. Electrically engaged overdrive gearboxes were an available option on all MGBs. The overdrive unit was operational in third and fourth gears (until 1977, when overdrive was only operational in fourth) but the overall ratio in third gear overdrive was roughly the same as fourth gear direct. The overdrive unit was engaged by a toggle switch on the dashboard or by pulling the wiper lever towards the driver, model dependent. The switch was moved to the top of the gearshift knob in 1977. Overdrives were fitted to less than 20% of all MGBs. There were three different types of overdrive transmissions fitted to the MGB. 1962–64, 1965–67 Laycock Type D OD (note external solenoid) A hole in the bell housing where the starter nose poked through "Shield" shaped access cover 1020 TPM for OD and 1040 TPM for non-OD The gearbox input shaft, flywheel and engine backing plate were changed with the advent of the five-main-bearing engine in 1965. Therefore, the transmission for a three-main-bearing engine (1962–1964) differed from its later counterpart. 1968 to 1974.5 Laycock Type LH OD Rectangular shaped access cover Oval clutch fork boot Dipstick (for checking oil) Black label on the OD solenoid cover stamped "22/61972" 1280 TPM for OD and non-OD Speedometer drive gear (on the mainshaft) was blue Speedometer driven gear (on removable drive housing) was white with 21 teeth 1974.5 to 1980 Laycock Type LH OD Rectangular shaped access cover Square clutch fork boot Side fill plug (no dipstick) Blue label on the OD solenoid cover stamped "22/62005" 1000 TPM for OD and non-OD Speedometer drive gear (on the mainshaft) was red Speedometer driven gear (on removable drive housing) was red with 20 teeth Overdrive operated in fourth gear only in units made from February 1977 onward. Early MGBs used the "banjo" type differential carried over from the MGA with the rear axle ratio reduced from the MGA's 4.1 (or 4.3) to 3.9 to 1. (Compensating for the reduction from 15 inch to wheels). MGB GTs first began using a tube-type rear axle in 1967. This unit was substantially stronger, being, like the later gearbox, designed for the three-litre MGC. All MGBs used the tube-type axle from 1968. All MGBs were fitted with solid (non-ventilated) disc brakes on the front with drum brakes on the rear. The front brake calipers