We talk numbers a lot in the car world. Horsepower, torque, quarter-mile times. But one set of numbers often gets thrown around without everyone really knowing what they mean: Gear Ratios.
You often hear it referenced as: “I’m running 373s,” or “I just swapped to 410s.” But what is actually happening inside that pumpkin to make those numbers reality?
Today, we are breaking down the simple math behind calculating your gear ratio – this is the foolproof way to know exactly what you are running.
The Formula: Ring Divided By Pinion
It might sound complicated, but finding your gear ratio is actually just simple division. Your rear differential has two main gears that interact:
- The Ring Gear: The large, plate-sized gear attached to the carrier.
- The Pinion Gear: The smaller, cone-shaped gear attached to the yoke and driveshaft.
The ratio is simply the number of teeth on the ring gear divided by the number of teeth on the pinion gear.
The Formula: Ring Gear Teeth ÷ Pinion Gear Teeth = Gear Ratio
How To Calculate Gear Ratio, Step-By-Step
If you have your third member on a bench or the cover popped off your diff, you don’t need to guess. You just need a marker and a finger to count.
- Step 1: Count the Ring Gear – Take a paint marker or a piece of chalk and mark one tooth on the ring gear as your starting point. Rotate the gear and count every tooth until you get back to your mark. In our video example, we counted 35 teeth on the ring gear.
- Step 2: Count the Pinion Gear – This one is a little harder to see since it’s tucked inside, but the process is the same. Mark a tooth on the pinion, spin it, and count. In our example, we counted 9 teeth on the pinion gear.
- Step 3: Do the math and plug those numbers into our formula: 35 (Ring) ÷ 9 (Pinion) = 3.888…
Round that up and you have a 3.89:1 gear ratio. This means your driveshaft has to spin almost 4 times (3.89 times, to be exact) to turn your rear wheels one full rotation.
What Does The Ratio Mean For Performance?
So, you know you have a 3.89, a 4.10, or the like, but how does that feel in the driver’s seat?
- Higher Numbers: (3.73, 4.10, 4.56): These are ‘shorter’ gears. They give you more mechanical leverage, which equals more bottom-end acceleration. If you want to launch hard off the line at the drag strip, you generally want a higher numerical gear ratio. The trade-off? You sacrifice top-end speed and your engine will scream at higher RPMs during highway cruising.
- Lower Numbers: (2.73, 3.08, 3.23): These are ‘taller’ gears. They are great for highway cruising and fuel economy because they keep your RPMs low, but your car will feel more sluggish off the line.
Re-Gear Or Upgrade With Moser Engineering
Knowing your ratio is step one. Having a rear end that can handle the power is step two.
At Moser Engineering, we don’t just sell parts; we engineer solutions for enthusiasts who break things. Whether you are looking to change your ratio for better 60-foot times or you need a bulletproof housing for your high-horsepower build, we have you covered:
- MUSCLE PAKS: Get a complete, bolt-in rear end assembly that installs using your OEM mounting points. No fabrication required – just bolt it in, fill it with fluid, and go.
- 100% New Parts: We never use remanufactured components. Every housing, axle, and gear set is 100% brand new and built in the USA.
- Customization: From spline counts to powder coating, we build it your way.
- Fastest Turnaround: We know you can’t win races if your car is on jack stands. We offer the fastest turnaround times in the industry to get you back on the track.
Don’t guess with your gears – put our four decades plus of experience to use. Get in touch with us if you have questions about your setup, or order online today!