What is differential gearing?

When talking about rear wheel drive BMWs, rear end gearing refers to the gears found in the rear end differential.  These gears come in different ratios, which greatly affect the performance of the car.

What is a rear differential?

The rear differential is the “box” where the rear drive shaft meets the two rear axles.  The differential allows the rear wheels to rotate at different speeds, important when turning as the outer wheels spin more than the inner wheels.  Vehicles without a rear differential, such as most go karts, allow both wheels to spin at the same speed.

Importance of differential gearing:

Rear end gear affects your cars acceleration, top speed and gas mileage.

What is gear ratios referring to?

Rear end gear ratios or final drive ratio refers to the number of revolutions the drive shaft turns compared to one full revolution of rear wheels.  A 2.90 gear ratio means the drive shaft turns 2.9 times in one full turn of the rear wheels.

What are tall and short gear ratios?

Tall gear ratios have LOW drive shaft rotations and short gear ratios have HIGH drive shaft rotations per one rotation of rear wheel.  Low drive shaft ratios are ideal for top speed and higher drive shaft ratios are ideal for acceleration.

An easy way to remember this is to imagine riding a 10 speed bike.  In first gear you are peddling like a mad man around town.  Your legs or “drive axle” is spinning considerably more than your rear wheel.  This short gear allows minimal power for quick acceleration but is not ideal for reaching top speeds.

Now imagine riding down a long straight road cruising for top speed, you want to be in a higher gear to lower your leg rotations in relation to the rear wheel.  This longer gearing is ideal for top speed but not for acceleration.

You can see how knowing the proper ratios for your car is critical to the application you are using it in.  Drag racing, top speed, rock crawlers, towing, race tracks, city driving all have ideal rear end ratios that will make your car perform differently in each situation.  

Changing gear ratios as a popular upgrade.

BMW offers many different rear end ratios on the same vehicle for different applications.  BMWs may differ in rear end gearing depending on the power output of the engine as well as if the car was a manual or automatic transmission.  By increasing your final drive ratio (i.e.: lower gears on a mountain bike) you are essentially increasing the RPM of your motor in relation to the speed of your car traveling down the road.  If your car makes peak power at 6k RPM but driving around town you never get past 4k RPM, you can change your differential gearing to reach this peak power RPM while still being within a desired speed range.

What are some BMW final drive ratios?

➛ E46 M3 Coupe, Manual Transmission: 3.62
➝ E90 325i Sedan, Manual Transmission: 3.23
➛ E90 335i Coupe, Manual Transmission: 3.08
➝ E36 318i Sedan, Automatic Transmission: 4.44
➛ E36 M3 Coupe, Manual transmission: 3.15

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