📐 Tire Size Comparison & Speedometer Error
Compare your current and a new tire size to see the diameter difference and exactly how much your speedometer will read off after the change.
| Spec | Original | New | Δ |
|---|---|---|---|
| Diameter | 24.97″ | 25.72″ | +2.98% |
| Sidewall | 101 mm | 98 mm | -3 mm |
| Revs / mile | 808 | 784 | -23 |
With the new tires, at an indicated 60 mph you're actually doing
61.8 mph
speedometer error reads slow by 2.98%
A larger overall diameter makes your true speed higher than the speedometer shows (and the odometer under-count). Keep a size change within about ±3% of diameter for accurate readings and clearance. 🔒 In your browser.
How the tire size comparison & speedometer error works
The tool computes each tire’s overall diameter and circumference from its code, then compares them. Because the speedometer is calibrated to the original tire, a larger new tire makes the wheels turn more slowly for the same road speed — so your true speed is higher than the reading. The true speed equals the indicated speed times the ratio of new to old diameter.
A rule of thumb is to keep a tire change within about ±3% of the original overall diameter — beyond that, the speedometer and odometer errors grow and you may run into clearance or gearing issues. This tool shows the exact percentage.
Frequently asked questions
How do different tire sizes affect my speedometer?
Your speedometer is calibrated for the original tire diameter. A larger new tire covers more ground per revolution, so at a given wheel speed you’re actually going faster than the speedometer shows; a smaller tire does the opposite. The tool gives the exact error.
How much bigger tire can I fit without recalibrating?
A common guideline is to stay within about ±3% of the original overall diameter. Larger changes noticeably skew the speedometer and odometer and can affect clearance, gearing and ABS/traction systems.
Do bigger tires make the speedometer read high or low?
Low. With larger tires you travel farther per wheel revolution, so your true speed is higher than the indicated speed — the speedometer reads slower than you’re actually going.
How do I keep my odometer accurate after changing tire size?
Match the new tire’s overall diameter to the original as closely as possible, or have the speedometer/odometer recalibrated (many vehicles allow this via a tuner or dealer). The tool’s diameter-difference percentage tells you how far off you’ll be.
What is a plus-size or minus-size tire fitment?
Plus-sizing fits a larger-diameter wheel with a lower-profile tire to keep the overall diameter about the same (e.g. 225/45R17 to 245/40R18). The goal is a similar rolling diameter — compare the two here to check.