How to Calculate Tire Size & Speedometer Error
Simple workflow instructions:
STEP 01
Enter Stock Tire
Enter OEM width, aspect ratio, and rim size (e.g. 225/45R17).
STEP 02
Enter New Tire
Enter new custom wheel/tire specs (e.g. 245/40R18).
STEP 03
Check Diameter
Ensure overall diameter difference remains within ±3% for ABS and speedometer accuracy.
STEP 04
Read Speed Error
Read actual vehicle speed when your dashboard speedometer reads 60 mph.
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Frequently Asked Questions
?What is the maximum safe tire diameter difference?
Automotive engineers recommend keeping new tire overall diameter within 3% of the factory stock tire size to prevent brake ABS errors and wheel well rubbing.