Tire Size Speedometer Error Calculator
See your real speed when you change tire size. Free, supports MPH / KM/H / knots, metric and imperial tire sizes. Results update as you type.
Calculate Your Speedometer Error
Enter your old and new tire sizes. Results update as you type.
Format auto-detected per field. Try 225/65R17 (metric), 33x12.50R17 (imperial flotation), 28.5 (inches), or 724 (millimetres). Mix freely.
Comparison at common speeds
| Indicated | Actual | Error |
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Odometer impact
Verify it for real with GPS
The calculator gives you the math. GPS Speedometer gives you the truth — your iPhone reading actual ground speed, no calibration, no tire-size dependence.
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How does this calculator work?
A factory speedometer doesn't measure ground speed directly. It counts how fast a wheel is rotating and multiplies that by the tire's expected circumference (set at the factory). When you swap to a tire with a different overall diameter, the wheel rotates at a different rate to cover the same ground — but the speedometer still uses the original circumference for its math, so the displayed speed drifts from reality.
The math is straightforward:
actual_speed = indicated_speed × (new_diameter / old_diameter)
If your new tire is 5% larger in diameter, the speedometer reads 5% slow at every speed: 60 indicated = 63 actual. The same ratio applies to the odometer — every mile counted is actually 1.05 miles travelled.
GPS sidesteps the problem entirely. A GPS receiver computes speed from position changes over time (latitude / longitude every fraction of a second), not from anything spinning under the car. Tire size, wear, slip, and gear ratios all become irrelevant.
Why does your speedometer lie?
Most jurisdictions allow factory speedometers to read 0–10% high (over-stating speed) but never low. The European ECE R39 regulation, the FMVSS 101 in the US, and equivalent standards elsewhere are all built around this asymmetry: the law would rather you think you're going slightly faster than you are than the other way around.
Combined with tolerances on tire dimensions and the wear-induced shrinkage of tires over their lifetime, a stock speedometer rolling on stock tires is typically 2–5% high. Swap to oversized tires and the speedo can flip below zero error — actually reading slow, which is illegal in some regions and dangerous everywhere because the driver underestimates real speed.
GPS speedometers carry no such bias. They report the truth as the GPS computes it, typically within 1–2% of actual speed at highway pace.
How do you read tire size notation?
There are three common formats you'll encounter:
225/65R17225 mm tread width, sidewall is 65% of tread width (so ~146 mm), R17 means a 17-inch wheel. Diameter = 17 × 25.4 + 2 × 146 = ~724 mm (~28.5 in).
33x12.50R17The first number IS the overall diameter in inches (33 in). Width is 12.5 inches. R17 means a 17-inch wheel.
28.5 or 724Some catalogues just list the diameter in inches or millimetres. Use the "Direct" format selector at the top of the calculator.
Frequently asked questions
Yes. The math is purely geometric — it doesn't depend on make, model, or year. As long as your inputs (old and new tire diameters) are correct, the output is correct to within the precision of the inputs. The only assumption is that the factory speedometer was originally calibrated for the original tire size, which holds for every modern road vehicle.
Slightly. A tire near end-of-life is typically 1–2 mm smaller in diameter than new (3–6 mm of tread depth has worn off the radius). For daily driving, use the as-new diameter — the wear correction is smaller than other tolerances and easier to ignore than to model.
Yes, by exactly the same percentage. If your speedometer reads 5% slow at every speed, your odometer counts 5% slow at every distance. Over 100,000 indicated miles that's 5,000 actual miles you didn't see counted — directly relevant for warranty windows, lease mileage caps, and resale value.
Often, yes. Many vehicles drive ABS, stability control, and transmission shift points off the same wheel-speed sensors that feed the speedometer. A significant tire-size change (more than ~3%) can cause shift hunting, premature ABS activation, or stability-control flagging at thresholds it wasn't designed for. Either keep the change small or have the speedometer recalibrated via the OBD-II port if you're running noticeably oversized tires daily.
Check the driver-side door jamb sticker (US/Canada market) or the owner's manual. If your tires are still original they'll have the size stamped into the sidewall. The format varies by region but it always matches one of the three formats this calculator supports (P-metric, imperial flotation, or direct diameter).
Yes — the calculator writes your tire sizes, speed, and units into the page URL as you type, so just copy the address bar (or bookmark it) and send it. Whoever opens the link gets the calculator pre-filled with your exact setup. The link updates on the full calculator page, not inside the embedded version.
Yes. A proper plus-size keeps the overall diameter constant (bigger wheel, lower-profile tire), so this calculator shows ~0% error if you've done it right. If you punch in a "plus-size" pair and the calculator shows non-zero error, the rolling diameter actually changed and the speedometer will read off accordingly.
Yes. Free download from the App Store, no signup. Core features (live speed, trip distance, basic recording) are free forever. The calculator on this page is also free with no signup required.
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