The phrase "extra cog" means having someone or something unnecessary or not needed in a situation or process. It refers to an additional part that serves no purpose or isn't helpful in achieving the desired outcome. Full definition
The V8 still comes with a six - speed manual as standard, but the optional six - speed Sportshift gearbox has gone, replaced by a new seven - speed Sportshift II «box - the extra cog helping to mix improved acceleration with all new levels of shift quality and refinement. (evo.co.uk)
Firstly, that gives you an extra cog to play with compared to the Twingo's three - pedal manual, but secondly, it means fairly quick, smooth shifts - an alien concept to previous Fortwo owners. (evo.co.uk)
This could be due to aerodynamics, but just as likely it's the extra cog in the much - maligned (by me) nine - speed automatic in the Holden. (motoring.com.au)