six-speed manual transmissions (parts code prefix: 01E, identification codes: five-speed: CBD, CET, six-speed: CBL, CMG) were offered to European customers, but a five-speed manual was the only transmission available in North America; a four-speed automatic transmission (parts code prefix: 01F, identification code: CBF) was available as an option worldwide. Front and rear axle final drive ratio is either 4.111 or 3.889 (dependent on transmission type), and the rear axle (parts code prefix: 017, identification codes: AAT, AET) uses a driver-selectable electro-pneumatic-mechanical switchable diff lock. Audi's quattro four-wheel-drive system was standard on the C4 S4/S6; a first-generation Torsen-based system which used the "T-1" automatically biasing centre differential (ATB) was used on vehicles equipped with manual transmissions, while a transfer box based system was used on vehicles equipped with an automatic transmission. S4/S6 "Plus" models equipped with a 4.2-litre all-aluminium alloy V8 engine (parts code prefix: 077, identification code: ABH) became available to European customers as an option from October 1992, only with the six-speed manual gearbox, in either saloon or Avant body styles. C4 steering, brakes, wheels & tires All C4 S4s' came equipped as standard with a conventional fixed-assistance rack and pinion power assisted steering (PAS) system. An electronically controlled 'servotronic' system, which varied the level of assistance in accordance with the road speed, was available as an option. There were two different types of high performance brake systems on offer for the C4 S4. All S4s' initially had diameter by thick, radially ventilated disc brakes up front, each with an ATE single-piston sliding caliper inside a 'UFO' styled bell-shaped disc. With the introduction of the V8 engine, a new front brake setup became standard; again the discs used radial ventilation, but these front discs were now sized at by - with a conventionally mounted Lucas-Girling caliper in an external position over the disc. US models had smaller discs. The rear brakes, by and also radially ventilated - remained the same throughout C4 production; they used a Lucas-Girling single-piston sliding caliper with an integrated cable-actuated handbrake mechanism. A Bosch three-channel operation (four-channel sensing) Anti-lock Braking System (ABS) with a mechanical load sensing valve for the rear brakes was standard throughout production. For wheel options, the standard five-arm "Fuchs" forged alloy wheels were 8.0Jx16H2 ET40, and were shod with 225/50 R16 sized tyres. Several other tyre and wheel designs were optionally available: a 7.5Jx15H2 ET40 six-spoke with 215/60 R15 tyres, an