Tire Plus-Sizing & Speedometer Error
Compare stock and new tire sizes to calculate overall diameter change, speedometer error percentage, and whether your plus-sized fitment is within safe tolerance.
Results
What is it?
Tire plus-sizing is the practice of fitting a larger rim diameter with a lower-profile tire to maintain a similar overall diameter. This improves handling (stiffer sidewall) and aesthetics. The speedometer error calculator reveals how much your instrument reading drifts when the rolling circumference changes.
How to use
Enter the three-part size codes for both your stock and new tires (width in mm, aspect ratio, rim diameter in inches). The calculator computes overall diameter for each and shows the percentage difference � which directly equals your speedometer error. A positive value means your speedometer reads low (you are going faster than indicated).
Example scenario
Upgrading from 225/45R17 (636.1 mm OD) to 235/40R18 (648.2 mm OD) gives a +12.1 mm diameter change and +1.9% speedometer error. At an indicated 100 km/h, you are actually doing 101.9 km/h � within the �3% tolerance most OEMs consider safe for ABS and ESC.
Pro tip
Keep speedometer error within �3% to avoid ABS and traction control calibration issues. Many ECUs can be recalibrated via OBD software to correct the speedo after a tire size change. Plus-sizing beyond +1 rim inch typically requires fender clearance and offset checks to prevent rubbing.