Stick?

[Lemur]

Americans would probably burn 25% less petrol if they all used manual.

Epic math fail. Average U.S. vehicle gets somewhere between 20-15 mpg. Most estimates are that you lose one mpg for an auto tranny as opposed to a manual.

Much more rational to say that Americans might burn as much as 5% less petrol if they went manual. (Note, my sedan averages 27 mpg, so I would only average 28 by going manual. The more efficient the vehicle, the less your savings would be. The biggest consumers of gas are commercial trucks anyway, but we won't go there just yet.)

-edit-

Just to make clear, for your statement to be correct, we would need to average 3-4 mpg with auto and 4-5 with manual. Even Hummers get better mileage than that.