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Automotive
Speedometer Error
When changing tire size
Runs locally in your browser
Parameters
Results
- Diameter difference
- 5.76%
- At 100 mph on speedo
- ≈ 95 mph actual
- Diameter old/new
- 632 / 668 mm
Formula: diameter = 2×width×profile/100 + rim×25.4
How it works
Estimates speedometer and odometer error after changing tire size, when the overall wheel diameter differs from stock.
Who it's for: Car enthusiasts upsizing wheels, truck owners switching to all-terrain tires, and anyone verifying that a new tire size stays within safe limits.
Computes outer tire diameter from width (mm), aspect ratio (%), and rim size (inches) for both old and new setups.
Reports the percentage change in diameter and what your actual speed is when the speedometer reads 100 mph.
Uses the standard formula: diameter = 2 × width × profile ÷ 100 + rim × 25.4 mm.
How to use
- Fill in Width (old), Profile (old), and Rim (old) from your current tire sidewall (e.g., 205/55 R16).
- Enter Width (new), Profile (new), and Rim (new) for the tire and wheel combo you plan to install.
- Read Diameter difference to see how much larger or smaller the new wheel is, in percent.
- Check At 100 mph on speedo for your real road speed at a common reference point.
- Review Diameter old/new for the calculated outer diameters in millimeters.
Good to know
- Most manufacturers recommend staying within ±3% of stock diameter to avoid ABS, traction control, and clearance issues.
- Factory speedometers are often calibrated slightly high — compare against GPS for a baseline before changing tires.
- A larger diameter makes the speedometer read slow; a smaller diameter makes it read fast.
FAQ
- Will a bigger tire affect my odometer?
- Yes. A larger tire travels farther per revolution, so your odometer will under-report miles driven, and vice versa for smaller tires.