Speedometer, Gear Ratio, and Tire Size Calculator – 2.5L NA

Overview

This procedure helps you calculate your Porsche 944’s speedometer accuracy, determine maximum speed in each gear, and assess the impact of different gear sets, final drive ratios, or tire sizes. It’s essential for ensuring your speedometer readings are correct, especially when modifying tires, wheels, or transaxle components, or verifying performance. This guide is tailored for the Porsche 944 with the 2.5L naturally aspirated engine (1983–1985.5 and 1986–1988 models).

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Tools Required

• Tachometer (ensure it’s accurate, or use a strobotach/stroboscope for verification)
• Calculator or computer for performing calculations
• Pen and paper for recording data
• Graph paper or spreadsheet software for plotting speedometer error

Step-by-Step Procedure

Step 1: Gather Required Information

1. Locate your Porsche 944’s owner’s manual to find the gear ratios and final drive ratio for your specific model (1983–1985.5 or 1986–1988).
2. Inspect the sidewall of your rear tires (drive wheels) to note the tire specifications. For example, a tire marked “225/50VR16” indicates-
   Tread Width: 225 mm
   Aspect Ratio: 50
   Wheel Diameter: 16 inches
3. Verify your tachometer’s accuracy. If you suspect it’s incorrect, use a strobotach/stroboscope to check it or have a speedometer shop calibrate it.

Step 2: Calculate Tire Circumference

4. Calculate the sidewall height:
   Multiply the tread width (in mm) by the aspect ratio (as a decimal). For example, for a 225/50VR16 tire, sidewall height = 225 mm × 0.50 = 112.5 mm.
5. Convert the sidewall height to inches:
   Divide the sidewall height by 25.4 (mm per inch). For example, 112.5 mm ÷ 25.4 = 4.43 inches.
6. Calculate the total tire diameter:
   Add the wheel diameter (in inches) to twice the sidewall height (in inches). For example, 16 inches + (2 × 4.43 inches) = 24.86 inches.
7. Calculate the tire circumference:
   Multiply the total tire diameter by π (3.14159). For example, 24.86 inches × 3.14159 = 78.10 inches.

Step 3: Calculate Theoretical Speeds

8. Select a gear to test (e.g., 4th gear) and note its gear ratio from the owner’s manual.
9. Choose several engine RPMs that are easy to read on your tachometer (e.g., 2000, 3000, 4000 RPM).
10. Use the following formula to calculate theoretical speed in miles per hour (MPH) for each RPM:
    Speed (MPH) = (RPM × Tire Circumference (inches) × 60) ÷ (Gear Ratio × Final Drive Ratio × 63360)
    For example, with 3000 RPM, 78.10-inch tire circumference, 0.83 gear ratio (4th gear), and 3.89 final drive ratio:
    Speed = (3000 × 78.10 × 60) ÷ (0.83 × 3.89 × 63360) = 68.78 MPH (110.72 km/h).
11. Repeat the calculation for each selected RPM to obtain a range of theoretical speeds.

Step 4: Record Indicated Speeds

12. Drive your Porsche 944 on a flat, safe road.
13. In the same gear used for calculations (e.g., 4th gear), maintain each selected RPM (e.g., 2000, 3000, 4000 RPM) and note the speedometer’s indicated speed in MPH and km/h.
14. Record these indicated speeds alongside the corresponding RPMs for comparison with theoretical speeds.

Step 5: Analyze Speedometer Error

15. Create a graph or use spreadsheet software to plot two lines: One line for theoretical speeds (calculated in Step 3) and one line for indicated speeds (recorded in Step 4).
16. Compare the two lines to identify the speedometer error across the RPM range. The difference between theoretical and indicated speeds indicates the error in MPH or km/h.
17. If the error is significant, consult a speedometer shop for calibration or consider adjustments to tire size or gear ratios.

Post Procedure Checklist

• Verify that all calculations are correct by rechecking tire circumference and speed formulas.
• Ensure recorded indicated speeds were taken at steady RPMs and on a flat road.
• Confirm the tachometer is accurate to avoid skewed results.
• Check that the plotted graph accurately reflects theoretical and indicated speeds.

Sources

• Clark’s Garage
• Pelican Parts
• Porsche 944 Workshop Manuals
• YouTube