Virtually no other factor influences driving pleasure and the sensation of engine power more directly than the ideal axle ratio. However, the ECE test cycle used to determine the respective CO2 emissions is driving the trend among vehicle manufacturers for longer rear-axle ratios as a means of complying with increasingly stringent CO2 limits. In many cases, a vehicle's dynamic performance suffers as a result.
Parameters that deliver theoretical fuel consumption benefits on the test bench generally have little to do with real-life driving conditions. On specific vehicles, a shorter rear axle ratio can even achieve better fuel consumption figures because the need for drivers to downshift – several times in the case of the eight-speed automatic transmission – when accelerating is eliminated.
<p">With this in mind, noelle motors has developed a shorter rear axle ratio with an electronic locking differential for the current BMW M5/M6 with the V8 bi-turbo engine and 7-speed DCT transmission. The ratio is reduced from the standard 3.15 to 3.64, thus turning seventh gear into a "driving gear" and enabling a theoretical maximum speed of 370 km/h at an engine speed of 7200 rpm.
A sports rear-axle differential is also available for the current BMW 550i, 650i and 750i models (without xDrive) equipped with the V8 bi-turbo valvetronic engine (N63TUE) and eight-speed automatic transmission. In this case, the ratio is reduced from the standard 2.81 to 3.46. As a result, Vmax is now reached in eighth gear rather than seventh, offering a theoretical maximum speed of 361 km/h at 6,500 rpm.