Harmony. The gathering of contributions from many sources to form a satisfying whole. It’s the goal of every car builder, and it’s magic when all the pieces fit. Engines, clutches, transmissions, drive shafts, axles, tires and suspensions, working together.

It doesn’t happen by accident. It takes planning. And there are bits of the puzzle your brother-in-law can’t help you with. Which manual transmission gearset to install, for example. But if you keep a simple ratio in mind you’ll find a piece that fits.

The magic ratio is 9:1. It’s the numerical target Chevrolet used when pairing gearboxes and axles, the result of multiplying the gearbox low ratio by the axle ratio. It’s not perfect — it needs to be modified up or down for various power curves, tire sizes or applications — but for cars with naturally aspirated engines and 27 inch rolling diameter tires in normal street service, it works.

For example, Chevrolet commonly paired a ‘Muncie’ M21 or M22 (2.20:1 low) with a 4.11 axle for a total reduction of 9.04:1, and a ‘Muncie’ M20 (2.52:1 low) with a 3.54 axle for a total reduction of 8.92:1. Start by selecting an axle ratio to provide a satisfying highway speed at a satisfying engine rev. Next, find a transmission gearset whose low gear multiplied by the axle ratio approximates 9:1. Then, identify the intermediate gears that best fit your driving requirement — move them up for the twisty-turnies, down for more acceleration at the strip. Need a bit more kick in your giddyup for your Camaro? Drop the axle ratio one step — a 3.73 with an M20 for 9.40:1. Got an Impala? You may need that 3.73 just to launch that additional weight.

Road racing, hill climbs, off-highway, truck and land speed applications, and super- or turbocharged power present unusual problems and opportunities — they’re why we offer 36 different gearset for our Syracuse M23’s. For these, or if you’re still unsure, we’re always ready to help. Give us a call!

These ratios are typical examples used by builders when applying the 9:1 rule. Variations exist across spline counts and specific applications.