Rear Gears for gas mileage

The rear differential (amongst other things) acts as a torque multiplier.

You have an 11 tooth input pinion gear attached to the yoke, and a 39 tooth output ring gear attached to the axles (3.545 output revolutions, for one input revolution). This ratio is rounded up to 3.55:1.

The higher the number (3.55 in this case), the higher the torque. This helps the car accelerate quicker. The trade off is a lower top speed in any particular gear (you reach redline quicker), and higher engine revs at a particular speed (compared to a different ratio, like 2.71:1).

You can equate this to your push bike. Big rear sprocket, more acceleration, less top speed. Small rear sprocket, less acceleration, more top speed.