E30 Gear Ratio Calculator: Speed & RPM

This calculator works out the road speed for each gear in a BMW E30 (or any manual car) from your gear ratios, final drive ratio, tire size, and engine RPM. Enter your transmission’s gear ratios, differential ratio, tire diameter, and RPM, and it returns the speed in each gear in km/h and mph, plus the speed per 1000 RPM.

Formula

The speed in each gear is calculated with this formula:

Speed (km/h) = (Engine RPM / (Gear Ratio × Final Drive Ratio)) × (Tire Diameter (inches) × 25.4 × π / 1000) × 60 / 1000

Where:

• Engine RPM – crankshaft revolutions per minute (unit: rpm)
• Gear Ratio – transmission gear ratio for the selected gear (unit: dimensionless, e.g., 3.83:1)
• Final Drive Ratio – differential ratio (unit: dimensionless, e.g., 4.10:1)
• Tire Diameter – overall diameter of the tire in inches (unit: inch, 1 inch = 25.4 mm)
• Ï€ – 3.14159
• 60 / 1000 – converts minutes to hours and meters to kilometers

The logic: engine RPM is divided by the total gear reduction (gear × final drive) to get wheel RPM. Multiply by the tire circumference (π × diameter) to get distance per minute, then convert to km/h.

Worked Example

Using the default values: 1st gear 3.83, 2nd 2.20, 3rd 1.40, 4th 1.00, 5th 0.81, final drive 4.10, tire diameter 23.0 inches, engine speed 6500 rpm.

1. 1st gear: 6500 / (3.83 × 4.10) = 6500 / 15.703 = 413.9 wheel rpm. Tire circumference = 23.0 × 25.4 × π / 1000 = 1.835 m. Distance per minute = 413.9 × 1.835 = 759.5 m/min. Speed = 759.5 × 60 / 1000 = 45.6 km/h.
2. 2nd gear: 6500 / (2.20 × 4.10) = 720.5 wheel rpm. Speed = 720.5 × 1.835 × 60 / 1000 = 79.3 km/h.
3. 3rd gear: 6500 / (1.40 × 4.10) = 1132.2 wheel rpm. Speed = 1132.2 × 1.835 × 60 / 1000 = 124.7 km/h.
4. 4th gear: 6500 / (1.00 × 4.10) = 1585.4 wheel rpm. Speed = 1585.4 × 1.835 × 60 / 1000 = 174.5 km/h.
5. 5th gear: 6500 / (0.81 × 4.10) = 1957.3 wheel rpm. Speed = 1957.3 × 1.835 × 60 / 1000 = 215.5 km/h.

These match the calculator output (rounded to 1 decimal).

Why E30 Owners Need This

The BMW E30 (1982–1994) came with many transmissions and differentials. Common gearboxes include the Getrag 240 (4-cylinder) and Getrag 260 (6-cylinder), with ratios that vary by year and engine. Final drive ratios range from 3.45 to 4.27 depending on model and market. Knowing your exact ratios helps you:

• Estimate top speed in each gear before a track day.
• Plan gear changes for maximum acceleration.
• Check if a swapped transmission or diff changes your speedometer accuracy.
• Compare the effect of a 3.73 vs 4.10 diff on highway RPM.

Most E30s use a 5-speed manual, but some automatics have different ratios. This calculator works for any car – just enter your own numbers.

How to Find Your Gear Ratios

You can find the ratios from:

• The vehicle’s factory service manual or owner’s manual.
• Online E30 forums and technical articles (search for your transmission code, e.g., Getrag 260/5).
• Counting revolutions: jack up the rear, mark the driveshaft and wheel, turn the wheel one revolution and count driveshaft turns – that gives the final drive ratio (approx).
• For transmission ratios, you can remove the gearbox and count teeth, but that’s rarely practical.

Common E30 5-speed ratios (Getrag 260, late 325i): 1st 3.83, 2nd 2.20, 3rd 1.40, 4th 1.00, 5th 0.81. Final drive for a 325i is often 4.10 (manual) or 3.73 (automatic). But verify for your specific car – production changes and swaps are common.

How to Read the Results

The first output (speed in 1st) is the headline number. It tells you the maximum speed in 1st gear at the given RPM. The other outputs show each gear. To use it:

• Set RPM to your redline (e.g., 6500 for M20 engines) to see top speed per gear.
• Set RPM to 3000 to see highway cruising speed in 5th – useful for checking if your tachometer matches reality.
• Compare two gear sets by changing ratios and noting the speed difference.

A common misconception: a taller (numerically lower) final drive like 3.73 gives higher top speed but slower acceleration. A shorter (higher) 4.27 gives quicker acceleration but lower top speed and higher highway RPM. This calculator shows that trade-off clearly.

Common Mistakes and Limits

• Wrong tire diameter: Use the actual diameter, not the rim size. A 205/60R14 tire is about 23.7 inches, but a 225/50R15 is about 23.9 inches. Check your tire sidewall or measure it.
• Mixing units: The formula uses inches for tire and km/h for speed. If you prefer mph, divide km/h by 1.609.
• Ignoring tire deflection: Under load, tire diameter shrinks slightly (maybe 3–5%), so real speed is a bit lower than calculated.
• Speedometer error: The factory speedometer often reads 5–10% high. This calculator gives true speed, not what your gauge shows.
• Redline variations: M10, M20, M40, M42, M50 engines have different redlines. Use the correct one for your engine.

The formula assumes no slip (clutch fully engaged) and a rigid tire. It’s accurate for planning, but not for official speed records.

Alternatives and Related Calculators

If you need to work backwards – find the RPM at a given speed – rearrange the formula: RPM = (Speed × Gear × Final Drive × 1000) / (Tire Diameter × 25.4 × π × 60). You can also use an online RPM calculator, but this one gives all gears at once.

For engine swaps, you might want to compare your new transmission’s ratios with the stock one. Enter both sets and compare the speed per gear. Some owners also use this to calibrate a GPS speedometer or a digital dash.

Remember: this calculator works for any manual car, not just the E30. The same math applies to a Honda, Porsche, or truck.

Frequently Asked Questions

What is the stock final drive ratio on an E30 325i?

Most manual 325i models (1987–1991) came with a 4.10 final drive, while automatics used 3.73. However, some markets and special editions (like the 325iS) had 3.73 or 3.91. Always check your differential tag – it’s stamped with the ratio.

Will a 3.73 diff make my speedometer read wrong?

Yes, if you change the final drive without recalibrating the speedometer. The speedometer reads from the output flange or the differential, so a different ratio changes the signal. This calculator helps you know your true speed, but to fix the gauge you need a speedometer gear or electronic correction.

How do I convert km/h to mph with this calculator?

Divide the km/h result by 1.609. For example, 100 km/h is about 62 mph. You can also change the tire diameter to inches and mentally convert, but the calculator only outputs km/h. For mph, just do the division.

Why does my 5th gear speed seem too high?

If your 5th gear ratio is very low (like 0.81), the engine turns fewer revolutions per wheel turn, so speed is high. But if your tire diameter is larger than stock, the speed increases further. Check your tire size – a 24-inch tire vs 23-inch adds about 4% speed.

What is the best final drive for an E30 track car?

It depends on the track and engine. A 4.10 is a common upgrade for acceleration, while 3.73 is better for high-speed circuits. Use this calculator to see where you hit the redline in top gear – if you never reach it, you need a shorter diff; if you hit it too early, a taller one.

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in g1 | 1st gear ratio | - | 3.83 | 0
in g2 | 2nd gear ratio | - | 2.20 | 0
in g3 | 3rd gear ratio | - | 1.40 | 0
in g4 | 4th gear ratio | - | 1.00 | 0
in g5 | 5th gear ratio | - | 0.81 | 0
in fd | Final drive ratio | - | 4.10 | 0
in tire | Tire diameter | inch | 23.0 | 0
in rpm | Engine speed | rpm | 6500 | 0
out spd1 | Speed in 1st | km/h | (rpm / (g1 * fd)) * (tire * 25.4 * pi / 1000) * 60 / 1000 | 1
out spd2 | Speed in 2nd | km/h | (rpm / (g2 * fd)) * (tire * 25.4 * pi / 1000) * 60 / 1000 | 1
out spd3 | Speed in 3rd | km/h | (rpm / (g3 * fd)) * (tire * 25.4 * pi / 1000) * 60 / 1000 | 1
out spd4 | Speed in 4th | km/h | (rpm / (g4 * fd)) * (tire * 25.4 * pi / 1000) * 60 / 1000 | 1
out spd5 | Speed in 5th | km/h | (rpm / (g5 * fd)) * (tire * 25.4 * pi / 1000) * 60 / 1000 | 1