Choosing a Gear Set 101

 

Spraker Racing Rear Axle Differential

First, let’s start with a few vocabulary words you’ll hear gear heads throw around.

  • Gear Ratio:  the ratio of the ring and pinion gears in the rear axle.  So, if you have a 4.10:1 (sometimes 4.10) rear axle, the pinion will turn 4.10 times for every single turn of the ring gear or in other words, for every 4.10 turns of the driveshaft, the rear wheel will spin once.  We explain how to determine your current gear ratio here.
  • Short Gear Sets:  the higher the numbers are (4.30, 4.11, 4.56, etc.) the shorter the gears.  Short gears require less power and accelerate quicker, but they aren’t very good for cruising on the interstate because the engine must maintain a very high RPM to stay at the higher speeds.  Short gears are great for quick acceleration and short drag strips.
  • Tall Gear Sets:  the lower the numbers are (3.25, 3.00, 2.90, etc.) the taller the gears.  These gears are great for interstate speeds, but they’re not great for accelerating from a dead stop.  Tall gears are made for longer drag strips and reaching top speeds.

 

Next, there are a couple of other factors to consider.

  • Tire height gives you the diameter of the tire.  A tire 21” tall is about 66” in circumference. (21” x π).  This means every time the tire turns, the car travels about 66”.  Taller tires travel greater distances for the same amount of drivetrain rotation.
  • The transmission transfers the rotation of the engine to the driveshaft.  The TKO 500 has a first gear with a 3.27 ratio.  This means the engine turns 3.27 times and the transmission spins the driveshaft once.  In first gear, your engine may be spinning at 3000 RPM, but because of the loss of rotation, the car only travels about 13.98 MPH.  On the other hand, the TKO’s overdrive gear is .68.  At 3000 RPM, the transmission spins the driveshaft one turn every .68 turns of the engine and the car will travel at 67.23 MPH.  (These equations assume a 4.10 axle and a 21” tire.)  An overdrive transmission can lessen the RPM load the engine must deliver to the rear axle, which will allow you to run a shorter rear axle without sacrificing your top speed.

 

How will your rear axle gear ratio affect performance?

Torque

In addition to turning the rotation of the drivetrain 90 degrees to spin the rear wheels, the rear gears multiply torque.  For instance, if your engine puts out 100 lb-ft of torque to a 4.10 rear axle, you can figure out how much torque the axle sends to the rear wheels by simply multiplying the engine torque with the rear axle ratio.  100x4.10=410 so the torque output will be 410 lb-ft.  If you need more torque then you may want to consider a shorter gear set.

RPM

After reading gears multiply torque and can make you accelerate really fast from a dead stop, you may think shorter gears are the way to go… Not so fast.  We have something else to consider - Engine RPM.  Using the same 4.10 gears, an engine will have to spin 4.10 times to turn a rear wheel once whether you’re going 10 miles per hour or 100.  This means the engine has to work really hard at faster speeds and in fact, your top speed may be very low.  So, if you’re frustrated by slow interstate speeds or because you have to stop for gas every 20 miles because your engine works so hard on the freeway, you may like a taller gear set.

Which gear is right for you?

Think about how you’ll be using the car and how it’s driving now.  The technical representatives at SST and our speed analyzer can help you figure out how different gear sets, transmissions, and tire heights will affect your speed and engine RPMs.  With this knowledge, you can improve the performance of your car, whether you are looking for better numbers at the track, gas pump, or somewhere in between.

 

Sources:

Gearing Lingo”. Hot Rod. TEN, 1 December 1998. Web. 2 September 2015.

Gears”. iRacing.com Wiki. n.p., n.d. Web. 2 September 2015.

Rear-End Gearing Simplified”. Bad Ass Racing Engines. Bad Ass Racing Engines. n.d. Web. 2 September 2015.