As was revealed recently, the Ford Motor Company has filed a patent in the United States that documents the development of an eleven-speed automatic gearbox in several configurations — a cool reference to Spinal Tap?

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But to understand it better we decided to take a look at the actual gear ratios to determine just what makes this 'box different to existing Ford units.

The patent filing, which was published this month, follows an announcement in early 2013 that Ford and GM were working on a joint venture to develop a ten-speed automatic gearbox, which will feature in the 2015 Ford F-150 Raptor.

Like most manufacturers, patent filings are created for everything from engineering concepts through to production designs. This particular application appears to be the former, given the lack of detail and reference to several potential configurations.

Key to the concept is a transmission that offers eleven forward gears and one reverse gear. The transmission is detailed in three configurations:

- One design includes four simple planetary gear sets, four clutches and two brakes.

- The second design includes two axis transfer gear pairs, three simple planetary gear sets, four clutches and two brakes.

- The third design includes four simple planetary gear sets, four brakes and two clutches.

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The reasoning behind the eleventh gear appears to be related to operating efficiency. The patent document claims that "some types of engines are capable of operating efficiently only within a narrow range of speeds". This indicates that the eleventh gear could be a ploy to extract further efficiency from an engine that has a narrow torque or efficiency band.

Compared to the ten-speed automatic created in the GM/Ford joint venture, the eleven-speed automatic in this patent filing aims to improve gearing ratios in higher gears to increase efficiency.

The ten-speed automatic gearbox operates at a 1:.636 ratio in tenth gear, while the eleven-speed automatic operates at 1:.67 in its tenth gear. When the eleven-speed automatic lobs into its eleventh gear, that ratio improves to 1:.58, presumably the efficiency advantage Ford engineers are chasing.

We will keep you posted if this new gearbox ever materialises beyond a concept.

In the interim, do you think we should draw a line in the sand somewhere when it comes to numbers of gears?