The numbers 3.55 and 3.73 refer to Ford truck axle ratios. The axle ratio tells you the number of revolutions the drive shaft makes compared to the rear axle. For a 3.55 axle ratio, the drive shaft will turn 3.55 times for every one revolution of the tire.
Ford Axle Ratios
Ford offers two axle ratios for their F-150 trucks; the 3.55 is standard and the 3.73 is optional. Ford offers different axle ratios to improve on hauling capability. The numerically higher number is the lower ratio.