DETROIT – There’s probably no better indication of the breakneck pace of automotive technical development than Mercedes-Benz’s new 7G-Tronic automatic transmission. The 7G-Tronic is the industry’s first automatic with seven forward ratios, yet its production-car launch last fall went practically unacknowledged.
Mercedes 7G-Tronic automatic transmission.

After all, it hasn’t been that long since 6-speed automatics hit the scene.

In a technical presentation for the 7G-Tronic at this week’s Society of Automotive Engineers World Congress here, Mercedes engineers say the new transmission improves both fuel economy and performance, yet its cost increase “is really small” when compared with the 5-speed automatic it replaces.

The 7G-Tronic achieves these gains largely through the adoption of the seven gear ratios, which provide a larger “spread” between the lowest and highest gears. Typically, the larger the ratio spread, the greater the potential efficiency gain.

But the 7G-Tronic also employs advanced features such as lockup of its smaller torque converter even for first gear – most automatics do not incorporate this efficiency-enhancing feature – and a unique control strategy that speeds downshifts by enabling the transmission to skip gears. A hard kick-down from seventh to third gear, for example, comes in just two steps – seventh to fifth, then fifth to third – rather that the four sequential downshifts a typical automatic would require.

Using Mercedes’ still-in-production 5-speed automatic for comparison, engineers say the 7G-Tronic improves city-cycle fuel economy 7.6% on average; highway fuel economy is enhanced by 7.2%. Despite these fuel-economy gains, wide-open-throttle acceleration is improved by 6% in a S500L sedan.

The 7G-Tronic is just 1.6 ins. (4.1 cm) longer than the current-production 5-speed automatic and its torque converter diameter is smaller, says Mercedes.

In North America, the 7G-Tronic is standard for V-8 powered S-, CL-, SL- and E-Class models. The V-8 equipped CLK-, G- and M-Class models continue with the 5-speed automatic says a Mercedes spokeswoman.

By the end of this year, Mercedes says it will be producing the 7G-Tronic at its Stuttgart, Germany, plant at the rate of 2,800 units daily.