Speedometer Error Calculator
Calculate the speedometer error caused by changing tire sizes. See a complete speed table showing indicated vs actual speed at every 10 mph.
Original Tire (OEM)
Format: 225/55R17
New Tire
Format: 235/65R17
Speedometer Error
+8.5%
Old Diameter
26.74โณ
New Diameter
29.03โณ
โ ๏ธ Speedometer reads SLOW. At 60 mph indicated, actual speed is 65.1 mph.
Speed Correction Table
| Speedo Shows | Actual Speed | Difference |
|---|---|---|
| 20 mph | 21.7 mph | +1.7 mph |
| 30 mph | 32.6 mph | +2.6 mph |
| 40 mph | 43.4 mph | +3.4 mph |
| 50 mph | 54.3 mph | +4.3 mph |
| 60 mph | 65.1 mph | +5.1 mph |
| 70 mph | 76.0 mph | +6.0 mph |
| 80 mph | 86.8 mph | +6.8 mph |
๐ก How to Use the Speedometer Error Calculator
How Tire Size Causes Speedometer Error
Your speedometer calculates speed by counting wheel rotations and multiplying by the factory tire circumference. When you change tire sizes, the actual distance per revolution changes, but the speedometer doesn't know โ it still uses the original calibration.
Understanding the Error Direction
- Taller tire โ Fewer rotations per mile โ Speedometer reads SLOW (you're going faster than indicated)
- Shorter tire โ More rotations per mile โ Speedometer reads FAST (you're going slower than indicated)
Legal & Safety Implications
A 3% speedometer error at highway speed means if your speedometer shows 65 mph, you're actually going 67 mph. In a 65 mph zone, that difference could mean a speeding ticket. For trucks with tachograph-based speed limiters, the error can cause the truck to exceed legal speed limits while the tachograph shows compliance.
How to Correct Speedometer Error
- Electronic recalibration: Most modern vehicles can have the ECU reprogrammed at a dealer ($50โ$150)
- Speedometer gear change: Older vehicles with cable-driven speedometers need a different driven gear
- GPS verification: Use a GPS app to verify your actual speed and calculate the error %