Calculating Auxiliary Transmission Gear Ratios: The Brownie and Compound Overdrive Explained

ByWyatt Brooks

Multiply the ratios — that’s the whole formula

Two transmissions in series work like a compound gear train: the output shaft of the first becomes the input of the second, and their ratios multiply. Main box in 0.80 overdrive, Brownie in 0.86 overdrive — multiply those and you get 0.80 × 0.86 = 0.688. That rounds to 0.69. The figure you sometimes see written as “0.68” is just a truncation, not a different calculation.

What the Brownie actually is

“Brownie” is the nickname for the Brown-Lipe auxiliary transmission, manufactured by Brown-Lipe-Chapin, a Syracuse, New York gear company that started supplying the early automobile industry. Their compact two-speed auxiliary units became a fixture behind medium and heavy commercial trucks through much of the mid-20th century.

A Brownie mounts between the main transmission and the driveshaft. Most variants offer three positions: underdrive, direct (1:1), and overdrive. Flip it into underdrive and you double your low-end range. Flip it into overdrive and you compound the top-end ratios. Specs varied across the production run, but typical Brown-Lipe units had an underdrive in the neighborhood of 1.28:1 and an overdrive somewhere between 0.73 and 0.86 depending on the model.

Working through the numbers

Main transmission top overdrive: 0.80. Brownie overdrive: 0.86. Combined:

0.80 × 0.86 = 0.688

In double overdrive, the effective transmission ratio is 0.688:1. The engine turns just 0.688 times per output shaft revolution — a real step below running the main box alone at 0.80:1. Whether that step is useful depends on what comes next in the drivetrain chain.

The full drivetrain chain

The compound transmission ratio is only part of the picture. Your true overall ratio — the one that determines actual engine RPM per wheel revolution — is the compound transmission ratio multiplied by your axle ratio.

Using a 3.73 rear axle as an example:

  • Main OD only: 0.80 × 3.73 = 2.98 effective final drive
  • Double OD: 0.688 × 3.73 = 2.57 effective final drive

That gap shows up directly on the tachometer. The difference between a 2.98 and a 2.57 final drive at highway speed is several hundred RPM — which matters for fuel economy, engine wear, and how loud the cab is on a long run.

Estimating cruise RPM

A practical approximation:

RPM = (Speed (mph) × Trans Ratio × Axle Ratio × 336) / Tire Diameter (inches)

The constant 336 comes from unit conversion: 63,360 inches per mile divided by 60 minutes per hour divided by π. Use your actual inflated and loaded tire diameter — the molded sidewall spec typically runs optimistic by an inch or more.

At 65 mph with 33-inch tires and a 3.73 axle:

  • Direct drive (trans ratio = 1.0): roughly 2,470 RPM
  • Main OD only (0.80): roughly 1,975 RPM
  • Double OD (0.688): roughly 1,700 RPM

These are approximations. But they tell you quickly whether double overdrive puts the engine in a comfortable range or drops it into territory where it’s working too hard on any grade.

When compound overdrive goes too deep

Deeper is not always better. Every engine has a floor — a minimum RPM below which it can’t hold load without lugging. If 0.688 compound OD drops you to 1,500 RPM and your engine’s peak torque lives at 1,800, you’ll be downshifting on every slight grade and erasing whatever fuel savings the overdrive was supposed to deliver.

Find where your engine makes peak torque and plan to cruise just above that number in top gear. That’s the target zone — relaxed enough to be kind to the drivetrain, but with enough RPM in reserve that a mild incline doesn’t immediately demand a kickdown.

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