Gear Vendor Calculator: How to Use It

A gear vendor calculator works out the road speed or gear ratio for a given engine RPM, transmission gear, and tire size. You enter the engine RPM, gear ratio, final drive ratio, and tire diameter, and it returns the vehicle speed in mph or km/h, plus the effective gear ratio in overdrive. This is the same math used by the Gear Vendors brand of auxiliary overdrive units, but the calculator itself is generic and works for any vehicle.

Formula

The calculator uses the standard vehicle speed equation:

Speed (mph) = (RPM × Tire Diameter × π) / (Gear Ratio × Final Drive × 1056)

Where:

• RPM = engine revolutions per minute
• Tire Diameter = overall tire height in inches (including the sidewall)
• Gear Ratio = the transmission gear ratio (e.g., 1.00 for direct drive, 0.68 for overdrive)
• Final Drive = the axle/differential ratio (e.g., 3.73)
• 1056 = a constant that converts inches, minutes, and revolutions to mph

The constant 1056 comes from: (12 inches/ft × 5280 ft/mile) / (π × 60 min/hr). It simplifies the unit conversions so you can plug in inches and get mph directly.

Worked Example

Using the default values from the calculator above:

• Engine RPM = 2500
• Gear ratio = 1.00 (direct drive)
• Final drive = 3.73
• Tire diameter = 31 inches

Step 1: Multiply RPM by tire diameter and π: 2500 × 31 × 3.14159 = 243,473.22

Step 2: Multiply gear ratio by final drive: 1.00 × 3.73 = 3.73

Step 3: Divide: 243,473.22 / 3.73 = 65,274.32

Step 4: Divide by 1056: 65,274.32 / 1056 = 61.8 mph

The calculator shows 61.8 mph, which rounds to 62 mph. In km/h, that is 61.8 × 1.60934 = 99.5 km/h.

What a Gear Vendor Unit Does

Gear Vendors is a brand of auxiliary overdrive that bolts to the back of the transmission (or transfer case in 4x4s). It adds an extra gear ratio—typically 0.78:1—so you get an effective overdrive when engaged. For example, with a 1:1 transmission gear and a 3.73 axle, the effective final drive becomes 3.73 × 0.78 = 2.91. That drops engine RPM by about 22% at the same speed, which reduces fuel consumption and engine wear on the highway.

The calculator above lets you compare speeds with and without the overdrive. Just enter the gear ratio as 1.00 for direct, then re-run with 0.78 to see the speed difference at the same RPM—or the RPM difference at the same speed.

How to Choose the Right Gear Ratio

When planning a gear vendor install, you need to know your current setup and your target cruising RPM. Common steps:

1. Find your transmission gear ratios (often stamped on the case or in the owner’s manual).
2. Find your axle ratio (check the door tag or count revolutions of the driveshaft).
3. Measure tire diameter (see below).
4. Decide your desired highway RPM at 65–70 mph (typically 1800–2200 for V8s, 2200–2600 for four-cylinders).
5. Use the calculator to test different final drive ratios (axle × 0.78) until the speed matches your target.

For example, if you want 70 mph at 2000 RPM with a 31-inch tire, you need an effective final drive of about 3.23. That means your axle ratio should be 3.23 / 0.78 = 4.14. So a 4.10 or 4.11 axle with the overdrive would work well.

Common Mistakes and Misconceptions

Mistake 1: Using tire radius instead of diameter. The formula needs the full height of the tire, not the radius. If you use radius, your speed will be half the real value.

Mistake 2: Mixing units. The 1056 constant only works with inches and mph. If you measure tire diameter in centimeters, you must convert to inches first (divide by 2.54). For metric speed, multiply the mph result by 1.60934.

Mistake 3: Forgetting the torque converter slip (automatic transmissions). The calculator assumes a 1:1 lockup. With a non-locking torque converter, actual RPM at speed can be 100–300 higher due to slip. For accurate results, use the RPM you actually see on the tachometer.

Misconception: Gear Vendors doubles your gears. It does not. It splits each gear, giving you a half-step between gears, but you still shift through the same transmission gears. The main benefit is the extra overdrive ratio for cruising.

How to Measure Tire Diameter

For the calculator to be accurate, you need the true tire diameter. The number on the sidewall (e.g., 31×10.50R15) is a nominal size, not always exact. Here’s how to measure:

• Park on level ground, set tire pressure to normal.
• Measure from the ground to the center of the wheel hub (this is the radius).
• Multiply that by 2 to get diameter.
• Alternatively, mark the tire and ground, roll one full revolution, and measure the distance traveled. Divide that by Ï€ to get diameter.

For metric tires like 265/70R17, convert: (265 × 0.70 × 2) / 25.4 + 17 = 31.6 inches. The first number is width in mm, the second is aspect ratio (sidewall height as a percentage of width).

Limitations of the Calculator

This calculator is a theoretical tool. It does not account for:

• Tire growth at speed (tires can expand slightly, increasing diameter).
• Transmission slip (torque converter, clutch).
• Vehicle weight, aerodynamics, or power—those affect whether the engine can actually maintain that speed at that RPM.
• Speedometer error—your speedometer may not be accurate even if the math is right.

For planning purposes, the result is usually within 1–2 mph of real-world speed, which is fine for gear selection.

Frequently Asked Questions

What is the difference between a gear vendor and a regular overdrive? A gear vendor is an auxiliary unit that adds a separate overdrive gear to your existing transmission. A regular overdrive is built into the transmission (like a 4th or 5th gear). The calculator works for both—just use the gear ratio of the overdrive gear you are using.

Can I use this calculator for a manual transmission? Yes. Manual transmissions have a 1:1 lockup in direct gear, so the formula is exact. For other gears, use the gear ratio from your manual’s specs.

How do I find my axle ratio without looking it up? Jack up the rear, mark the driveshaft and tire, turn the tire one full revolution and count how many times the driveshaft turns. That number is your axle ratio (e.g., 3.5 turns = 3.50:1). For limited-slip or locking diffs, both tires must turn; for open diffs, keep one tire on the ground.

Does tire pressure affect the calculation? Yes, slightly. Lower pressure reduces the rolling radius, which lowers your actual speed at a given RPM. The effect is usually less than 1 mph, but for precise planning, measure at your normal driving pressure.

Why does the calculator give a different speed than my GPS? GPS measures actual ground speed, while the calculator assumes no tire slip and exact tire diameter. Your speedometer may also be off. If the difference is more than 2–3 mph, re-measure your tire diameter and check your axle ratio.

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in rpm | Engine RPM | rpm | 2500 | 0
in gr | Gear ratio (e.g. 1:1 = 1) | - | 1 | 0
in fd | Final drive ratio | - | 3.73 | 0
in td | Tire diameter | inches | 31 | 1
out spd | Road speed | mph | (rpm * td * pi) / (gr * fd * 1056) | 1
out spd_kmh | Road speed | km/h | spd * 1.60934 | 1