Speedometer error after a tire size change
Short answer: if your new tires are taller than the originals, your speedometer reads lower than your real speed; if they are shorter, it reads higher. Your real speed is roughly the speedometer reading times the new diameter divided by the old diameter. Continental says a change within plus or minus 3 percent normally needs no adjustment.
Last updated: October 28, 2026
Speedometer error calculator
Enter your original and new tire sizes. This runs entirely in your browser and nothing is sent or stored.
Calculated from nominal sizes. Real tires vary slightly by brand, model and tread depth.
Why a tire size change affects the speedometer
Most speedometers work out speed from how fast the wheels or drivetrain turn, calibrated for the original tire size. A taller tire travels farther with each turn, so the car moves faster than the speedometer thinks. A shorter tire does the opposite. Continental notes that a different rolling circumference can also affect ABS and stability control, not only the speedometer.
The math
- Sidewall height in inches = width in mm times aspect ratio divided by 100, then divided by 25.4.
- Overall diameter = rim diameter plus two sidewalls.
- Real speed = speedometer reading times new diameter divided by old diameter.
Worked examples at an indicated 60 mph
| Original | New | Diameter | Change | Real speed |
|---|---|---|---|---|
| 225/65R17 | 235/65R17 | 28.52 in to 29.03 in | +1.8% | 61.1 mph |
| 205/55R16 | 225/45R17 | 24.88 in to 24.97 in | +0.4% | 60.2 mph |
| 265/70R17 | 285/75R17 | 31.61 in to 33.83 in | +7.0% | 64.2 mph |
| 225/65R17 | 265/70R17 | 28.52 in to 31.61 in | +10.8% | 66.5 mph |
The first two stay inside the plus or minus 3 percent range. The last two are well outside it, so the speedometer would read noticeably low and the change should be checked with the vehicle manufacturer. Size changes can also affect clearance and load capacity; see plus sizing and load index and speed rating.
Before you change sizes
- Start from the size on your door placard, which is what the speedometer was set up for.
- Keep the overall diameter as close to stock as you can, ideally within plus or minus 3 percent.
- Match or exceed the original load index and speed rating.
- Fit the same size on all four wheels unless the vehicle came with a staggered setup. See mixing tires.
- If you go outside the range, ask a dealer whether the speedometer can be recalibrated for the new size.
Compare width, sidewall and revolutions per mile as well in our tire size calculator, or read how to choose the right tire size.
Frequently asked questions
Do bigger tires make my speedometer read slower or faster?
Taller tires make the speedometer read lower than your real speed, because each turn of the tire covers more ground. Shorter tires make it read higher than your real speed. The size of the error depends on how much the overall diameter changes.
How do I work out my real speed after a tire size change?
Divide the new overall diameter by the old one and multiply by the speed on your speedometer. For example, going from 225/65R17 to 235/65R17 raises the diameter by about 1.8 percent, so an indicated 60 mph is about 61.1 mph.
How much can tire size change without affecting the speedometer?
Continental says that under most circumstances a change in rolling circumference within plus or minus 3 percent does not require adjusting the speedometer, ABS or stability control. Outside that range, or if in doubt, follow the vehicle manufacturer.
Does the odometer change too?
Yes. The odometer uses the same signal as the speedometer, so it records distance with the same percentage error. Taller tires make it count fewer miles than you actually drive; shorter tires make it count more.
Can a speedometer be recalibrated for new tires?
On many vehicles a dealer or specialist shop can update the tire size setting or recalibrate the speedometer. The method depends on the make and model, so ask the vehicle manufacturer or a dealer.
Does tread wear change the speedometer reading?
Slightly. As tread wears, the tire's diameter shrinks a little, so the error from a size change shifts a bit over the tire's life. The calculator here uses nominal sizes, which describe a new tire.
Sources
- Continental Tire, Plus sizing (PDF): a rolling circumference different from the original fitment can affect speedometer calibration, ABS and stability control; under most circumstances a change within plus or minus 3 percent needs no adjustment; ask the vehicle manufacturer if in doubt
- NHTSA, Tires: tire size, the placard and choosing replacement tires