The crown wheel and pinion is the bevel-gear pair inside the rear differential that turns the rotation of the propshaft, running fore-and-aft along the chassis, through 90 degrees to drive the rear axle, running across the car, with the ratio of the crown wheel to the pinion determining the final drive ratio, the number of engine revolutions required to turn the wheels through one revolution. Changing the final drive ratio is one of the most fundamental tuning steps available, transforming the character of the car between an acceleration-focused short-ratio setup and a cruising-focused long-ratio setup, with fitting a numerically lower, taller, final-drive ratio delivering improved top speed, lowered cruising rpm, reduced cabin noise, and better fuel economy for cars used regularly on motorways. The MGOC sets are manufactured in the UK from hardened steel with parallel bevel-cut pinion gears for increased durability, well suited to higher-torque applications.
Standard Final Drive Ratios
The classic-MG range used several different final drive ratios across production, the chrome-bumper MGB using a 3.909:1 ratio as standard with a 3.7:1 option for cars specified with overdrive, the MGC using a 3.31:1 ratio matched to its more torquey six-cylinder engine and the higher gearing this allowed, the Midget using various ratios across its production typically in the 4.22:1 to 4.55:1 range depending on the specific model year and gearbox arrangement, and the MGA using a 4.3:1 standard ratio with alternatives for competition applications. The range is manufactured to the original factory ratios including the 3.7:1 conversion ratio for Midget 1500 cars and the 3.07:1 ratio for MGC and MGB GT V8 cars retaining their original Salisbury rear axles. Each ratio is appropriate to the engine and gearbox combination of the car as originally specified, and changes to other drivetrain components, a gearbox conversion, engine modification, or tyre-size changes, typically suggest a corresponding change in the final drive ratio to keep the overall gearing balanced.
Alternative Ratios for Tuning
Alternative crown wheel and pinion sets allow the final drive to be changed to suit the engine, gearbox, and intended use, a taller ratio with a lower numerical value such as 3.5:1 in place of 3.909:1 reducing engine rpm at cruising speed and improving motorway refinement and fuel economy at the expense of slightly reduced acceleration, while a shorter ratio with a higher numerical value such as 4.22:1 in place of 3.909:1 raising engine rpm at any given speed and improving acceleration and engine response at the expense of motorway refinement. The choice depends on how the car is used, motorway-touring cars benefiting from taller ratios, cars used principally on B-roads and in town benefiting from shorter ratios, and competition cars often specified with very short ratios that maximise acceleration response, the wider range from around 3.07:1 to 4.55:1 covering most owner preferences. The sets are dimensioned to fit within a standard axle casing, which means it is not necessary to source a V8 axle to benefit from an alternative final drive ratio, making them a particularly popular choice for owners undertaking a V8 conversion using the original four-cylinder axle.
Installation Considerations
Crown wheel and pinion fitting is workshop-level work requiring specialist tooling and experience, the setup involving precise adjustment of the pinion depth, the distance from the centre of the crown wheel to the pinion-gear face, the crown-wheel backlash, the clearance between the crown-wheel teeth and the pinion teeth at the mesh, and the contact pattern, the area of the tooth face where the two gears actually transfer load. Setting these correctly requires dial-test indicators, specific gauges, and the experience to read the contact pattern accurately, so most owners commission the work from a specialist workshop rather than attempting it themselves, the resulting drivetrain, a properly set crown wheel and pinion paired with new bearings and oil seals, providing quiet, reliable operation through many years of subsequent service. Sets are available outright or can be supplied built up into a customer's own unit, and the technical team is available to advise on the right ratio selection and to discuss the supporting components, bearings, oil seals, and differential service items, needed for any final-drive change.