Speedometer, Gear Ratio, and Tire Size Calculator

Overview

This guide is designed for Porsche 944 (16v engine) owners to calculate speedometer accuracy and determine maximum speeds in each gear. Whether you’re checking for speedometer error, considering a tire size change, or evaluating your 944’s performance with its stock transaxle, this process uses your car’s specific gear ratios, final drive ratio, and tire dimensions. It assumes your tachometer is accurate—vital for reliable results.

Tools Required

• Tachometer (verify accuracy with a strobotach/stroboscope if unsure)
• Calculator or computer for the formulas
• Pen and paper to record speeds
• Optional: Graph paper or software to plot speedometer error

Step-by-Step Procedure

Step 1: Gather Your Porsche 944 Specifications

1. Locate your gear ratios and final drive ratio in the Porsche 944  owner’s manual. For the 944 (16v engine) with the standard 5-speed manual transaxle, these are:

1st Gear: 3.500:1

2nd Gear: 2.059:1

3rd Gear: 1.400:1

4th Gear: 1.034:1

5th Gear: 0.829:1

Final Drive Ratio: 3.89:1

Confirm these match your car, as modifications could alter them.

2. Check the tire size on the sidewall of your rear tires (the 944 is rear-wheel drive). The stock size is typically 225/50ZR16:

Tread Width: 225 mm

Aspect Ratio: 50

Wheel Diameter: 16 inches

Step 2: Calculate Tire Circumference

3. Using the tire details, calculate sidewall height: Aspect Ratio × Tread Width. For 225/50ZR16, that’s 50% of 225 mm = 112.5 mm.
4. Convert to inches: 112.5 mm ÷ 25.4 = 4.43 inches.
5. Add two sidewall heights (top and bottom) to the wheel diameter: 4.43 inches + 4.43 inches + 16 inches = 24.86 inches (tire diameter).
6. Calculate tire circumference: Tire Diameter × π (3.1416). So, 24.86 inches × 3.1416 = 78.1 inches.

Step 3: Calculate Theoretical Speeds

7. Choose easy-to-read RPMs on your 944’s tachometer (e.g., 2000, 3000, 4000 RPM). The 16v engine redlines at about 6800 RPM, so stay within safe limits.
8. Use this formula for speed in MPH: Speed = (RPM × Tire Circumference) ÷ (Gear Ratio × Final Drive Ratio × 336). For example, in 3rd gear (1.400:1) at 3000 RPM with a 78.1-inch circumference and 3.89:1 final drive:

Speed = (3000 × 78.1) ÷ (1.400 × 3.89 × 336) = 234,300 ÷ 1829.136 ≈ 128 MPH (206 km/h).

9. Repeat for other RPMs and gears as needed (e.g., 5th gear at 4000 RPM).

Step 4: Test Your Speedometer

10. Drive your Porsche 944 in the same gear used in Step 3 (e.g., 3rd). Hold the RPM steady at your chosen values (e.g., 2000, 3000, 4000 RPM).
11. Record the “indicated” speed on your speedometer at each RPM.

Step 5: Analyze Speedometer Error

12. Compare “theoretical” speeds (Step 3) to “indicated” speeds (Step 4). For example, if 3000 RPM in 3rd gear calculates to 128 MPH (206 km/h) but your speedometer shows 120 MPH (193 km/h), it’s reading 8 MPH (13 km/h) low.
13. Optional: Graph theoretical vs. indicated speeds to see the error trend across the RPM range.

Post Procedure Checklist

• Confirm tachometer accuracy during the test drive.
• Verify tire size and gear ratio inputs match your 944’s setup.
• Check that calculated speeds align with the 16v engine’s performance (e.g., top speed in 5th gear should be near 150 MPH / 241 km/h).

Sources

• Clark’s Garage
• Pelican Parts
• Porsche 944 Workshop Manual, Volume 1
• YouTube