A commonly overlooked component of a vehicle’s drivetrain that has a large effect on fuel economy is rear end gearing. This is the ratio of gears in the rearend (differential) that you may have heard of in the numerical terms 2.73 to one, 3.23 to one, 3.55 to one, 3.73 to one, and 4.10 to one (commonly written in the following manner 3.23:1 with a colon). These numbers specifically relate the ratio of the speed at which the pinion gear rotates to that of which the larger ring gear rotates within the differential. This means, for a 3.73 to 1 setup, that your driveshaft will spin the pinion gear a total of 3.73 times before one full revolution is completed with the ring gear which in turn spins the rear tires through the rear axles.
What does this all mean you ask? Well the bottom line is that the smaller the pinion gear or higher the numerical ratio i.e. 4.10 to 1 (a typically high ratio) means that the quicker your acceleration will be and the lower your car’s top speed that can be reached will be as well. You will be going through the transmission gears more quickly while driving and achieving a faster acceleration but will be losing fuel economy as well. For example with a 3.41 gear you maybe be at 1800 rpm in 6th gear on the highway at 80 miles per hour but with a 3.73 gear you would be at say 2100 rpm at the same speed 80 miles per hour in 6th gear. So you will be burning more fuel at the same highway speed because you are at a higher rpm. See the manual transmission section for an explanation of how one wastes more fuel at a higher rpm.
Power Gain & Fuel Economy
Installing higher rear end gear numbers will not technically increase your horsepower. In fact it will slightly decrease your horsepower and torque on a dyno test. However it will greatly increase your acceleration and allow your engine to operate at a higher rpm throughout your normal driving range. So you will be utilizing more of your engine’s horsepower and torque all the time. For example, typical LT1 and LS1 powered Camaros and Firebirds at the race track can reduce their quarter mile times by up to 2 or 3 tenths in the quarter mile by switching from 3.23 to 3.73 or 4.10 gears.
If you are looking to boost fuel economy, installing a set of lower numerical gears would be your best bet. Switching from a set of 3.23s to 2.73s will lower your auto’s normal operating rpm range during driving which will translate to higher fuel economy for you the driver. The difference can be anywhere from a few mpg to several mpgs depending on how much of a smaller ratio you select.
The fuel savings or increased acceleration advantages can be quite dramatic with a change in differential gearing. And you can select how much of a change you want with the large variety of sizes available on the market today from such rear end gear manufacturers as Richmond, Motive, and Strange Engineering to name a few. However the biggest drawback is in installation and labor. Installing a new set of rear end gears is commonly known as one of the most difficult automotive tasks to accomplish. It requires precise measurements when lining up the contact area of the two gears and must be redone several times if they are not lined up properly. If the gears do not mesh correctly during setup, this will lead to premature wearing of the gears, excess friction in the driveline, and whining sounds coming from the rear end while driving. Having said this it is wise to select a rear end specialist with many years of experience in installing differentials to perform this task.