What Is the Effect of Changing From a 2.59 to a 3.73 Rear Axle?

The rear axle in your vehicle contains the differential, which transmits power from the driveshaft to the axles. The main component in the differential is the ring and pinion gear. The ring and pinion gear are referred to by their ratio. Many vehicles are available from the factory with more than one axle ratio. Different gear ratios can have a direct impact on factors including acceleration, pulling power and fuel economy.

Gear Ratio

Gear ratio is determined by the number of teeth on the ring and pinion gears. For instance, a gear set with a ratio of 2.59-to-1 might have a ring gear with 44 teeth and a pinion with 17 teeth. The ratio is calculated by dividing the ring gear by the pinion gear; 44 divided by 17 equals 2.588, which is rounded up to 2.59. This means that for every 2.59 revolutions of the driveshaft, the rear wheel turns one time. A 3.73-to-1 gear set has 41 teeth on the ring gear and 11 on the pinion. Forty-one divided by 11 equals 3.727, rounded to 3.73.

High and Low Gears

Gear sets usually are put into two categories: high and low. It sounds backward and may be confusing, but gear sets with numerically smaller numbers such as 2.59-to-1-- are referred to as high, while those with a numerically larger numbers such as 3.73-to-1 are referred to as low. Higher gears tend to give better highway mileage than low gears because at any given engine speed, the engine is turning fewer rpm, but does not accelerate as quickly because fewer engine rpm is sent to the wheels. Low gears accelerate better because more engine rpm are used to turn the wheels at any given speed, but mileage is reduced because the engine is turning faster.

2.59-to-1

A 2.59-to-1 gear ratio generally is considered a high gear set. Off-the-line acceleration is inferior when compared to the 3.73-to-1 gear set because at any given time, in any given gear, fewer engine rpm are being used to turn the wheels. Fuel economy is superior with the 2.59-to-1 gears for the same reason: fewer engine rpm at any given speed. The 2.59-to-1 gears also provide a higher top speed. Engine rpm can be calculated with the following formula: (mph x axle ratio x 336) / tire diameter = rpm. This formula uses a 1-to-1 transmission gear ratio such as is common to third gear in a three-speed automatic, or fourth gear in a four-speed manual transmission. Using a tire diameter of 26 inches, a speed of 65 mph and the formula, you get 65 x 2.59 x 336 = 56,565.6 / 26 = 2,175.6, which is rounded to 2,176 rpm. To get the actual number for your vehicle, you have to know the diameter of your tire.

3.73-to-1

These gears offer better acceleration and pulling power because at any given speed and gear, more engine rpm is used to turn the wheels. Top speed is lower. Using the same formula, we get 3,133 rpm at 65 mph -- a difference of 857 rpm. If you do not want to do the math, there are handy calculators on the Internet.

Calculate for Any Gear

Each gear in your transmission also has a ratio. Lower gears such as first and second have ratios greater than 1-to-1. For instance, first gear may have a ratio of 3.5-to-1. Second gear may be 2.4-to-1. An overdrive gear like that of a four-speed automatic or five-speed manual is less than 1-to-1; for instance, 0.70-to-1. To calculate rpm for various gears, multiply the answer from the formula by the transmission gear ratio. Using the 3,133 rpm for the 3.73-to-1 gears and an overdrive gear, multiply 3,133 times the overdrive ratio. With a 0.70-to-1 overdrive, engine rpm would be 3,133 x 0.70 = 2,193.