What are 4.10s, and 2.73s, etc.??? Does it affect gas mileage, and anything else?

 

Those numbers are ratios. 2.73:, 4.10:1 and refer to the ratio of the number of teeth on the ring gear to the number on the pinion. The pinion is connected to the driveshaft and the ring gear to the axles (by way of the differential). So, if the ratio is 2.73:1 the axles will turn one revolution for every 2.73 revolutions of the driveshaft. If it's "4.10 rear end" the driveshaft has to turn 4.10 revolutions to drive the axles to rotate through one revolution.

With a (numerically) higher axle ratio the engine will turn (rev) faster for a given road speed and there will be more torque multiplication. But, of course, you will have to shift gears (in the transmission) sooner with a higher rear end ratio to avoid over-revving the motor. The higher the ratio the more wear on the engine and the worse the gas mileage is, in general. But there is the potential for faster acceleration provided there is adequate traction and the motor is operating within its' powerband.

Choosing the best rear end ratio for your particular application involves a lot of variables such as the weight of the vehicle, the amount of torque and the torque curve, the redline of the motor, the gear ratios in the transmission, tire size, intended use, etc.

Rich Krause