<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (July 2015) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (July 2015) (Learn how and when to remove this template message) </Td> </Tr> <P> A differential is a gear train with three shafts that has the property that the rotational speed of one shaft is the average of the speeds of the others, or a fixed multiple of that average . </P> <P> In automobiles and other wheeled vehicles, the differential allows the outer drive wheel to rotate faster than the inner drive wheel during a turn . This is necessary when the vehicle turns, making the wheel that is traveling around the outside of the turning curve roll farther and faster than the other . The average of the rotational speed of the two driving wheels equals the input rotational speed of the drive shaft . An increase in the speed of one wheel is balanced by a decrease in the speed of the other . </P>

The smaller gears inside the differential casing are