Calculate your speedometer error and true speed after a tire size change or from a GPS speed check, plus the odometer impact of new tires.
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Speedometer Error Formula
AS = IS * (ND / OD)
E% = ((IS - AS) / AS) * 100
- AS is your actual (true) road speed
- IS is the speed shown on the speedometer
- ND is the overall diameter of the new tire
- OD is the overall diameter of the original tire the speedometer was calibrated for
- E% is the speedometer error as a percentage of true speed
The tire-change model assumes the speedometer was accurate on the original tires and calibration remains unchanged. It multiplies the shown speed by ND/OD to estimate actual speed after the change. Nominal tire diameter is a geometric estimate of rolling size: construction, load, pressure and wear can change actual rolling circumference. Existing calibration error is not inferred from tire dimensions. The expression E% = (IS − AS)/AS × 100 requires positive actual speed. At standstill, tire mode reports the ratio-derived error for nonzero speeds.
For sidewall dimensions such as 225/65R17, the calculator first estimates nominal overall diameter without rounding intermediate values:
D = R + (2 * W * (A / 100)) / 25.4
- D is the overall tire diameter in inches
- R is the rim diameter in inches
- W is the section width in millimeters
- A is the aspect ratio (sidewall height as a percent of width)
The GPS mode compares a paired dash and GPS reading: E% = ((IS − GPS)/GPS) × 100. The observed reference/shown factor is GPS/IS at that point. Estimates at other readings assume the same multiplicative factor; one sample does not calibrate the full speed range. Odometer impact is calculated only from the tire-change model, assuming the odometer was also accurate on the original tires and still uses that calibration. Factory speedometer display bias need not be an odometer bias.
Speedometer Error by Tire Size Change and Accuracy Standards
The first table assumes accurate original calibration and shows the nominal diameter model at an indicated 60 mph. A positive diameter change means larger tires and a lower indicated speed relative to modeled actual speed. Diameter change and signed speedometer error use different denominators.
| Diameter change | True speed at indicated 60 mph | Speedometer error |
|---|---|---|
| -6% | 56.4 mph | Reads 6.4% high |
| -4% | 57.6 mph | Reads 4.2% high |
| -2% | 58.8 mph | Reads 2.0% high |
| 0% | 60.0 mph | No tire-related error |
| +2% | 61.2 mph | Reads 2.0% low |
| +4% | 62.4 mph | Reads 3.8% low |
| +6% | 63.6 mph | Reads 5.7% low |
| +8% | 64.8 mph | Reads 7.4% low |
The second table gives scoped references, not a compliance verdict from this calculator. UN Regulation 39 is shown as a published 2018 type-approval reference; its conditions, vehicle classes, current version and applicability must be checked for the relevant market. A tire estimate or single GPS pair does not establish a vehicle’s legal compliance.
| Standard | Applies to | Allowed error |
|---|---|---|
| UN Regulation 39, published 2018 reference | Relevant vehicle classes and prescribed approval-test conditions where applicable | At the prescribed tests, indicated speed is not below true speed and excess is at most 10% of true speed plus 4 km/h. Confirm current version and market applicability. |
| Vehicle/model and market specifications | Applicable passenger vehicles | Requirements depend on the vehicle, manufacturer specification and applicable jurisdiction. |
| 49 CFR 393.82 (FMCSA) | Buses, trucks and truck-tractors subject to these FMCSA rules | Accurate within plus or minus 5 mph at 50 mph |
| SAE J2976_202205 recommended practice | Its scope includes new OEM radial-tire passenger cars and light/medium trucks | A recommended practice, not a universal law. Consult the applicable version for numerical limits and test conditions. |
If the odometer was accurate on the original tires and calibration is unchanged, tires with 6% greater rolling diameter imply about 10,600 actual miles per 10,000 indicated miles. That is a distance ratio of 1.06; signed indicated-versus-actual distance error is (10,000 − 10,600)/10,600 ≈ −5.66%, rather than −6%. The estimate is not evidence of mileage for warranty, lease or resale purposes.
Example Problems
Example 1: Tire size change
You replace 225/65R17 tires with 245/70R17 tires and want your true speed at an indicated 60 mph.
Nominal original diameter is 17 + (2 × 225 × 0.65)/25.4 ≈ 28.515748 in; new diameter is 17 + (2 × 245 × 0.70)/25.4 ≈ 30.503937 in. Keep the full computed values when forming the ratio.
Using the unrounded diameters, the ratio is 7748/7243 ≈ 1.069722491. At an indicated 60 mph, estimated actual speed is 60 × (7748/7243) ≈ 64.183349 mph, or 64.2 mph to one decimal. Signed error is approximately −6.517811%, or −6.5% to one decimal. Under the same accurate-original-odometer assumption, 10,000 indicated miles represents approximately 10,697.224907 actual miles, or about 10,700 miles.
Example 2: GPS speed check
Your dash shows 70 mph while a GPS app reads a steady 66 mph.
The paired-reading error is (70 − 66)/66 × 100 ≈ +6.060606%, or +6.1% to one decimal. The unrounded factor is 66/70 = 33/35 ≈ 0.942857143. If the same proportional relationship holds at another speed, an indicated 60 mph corresponds to 60 × (66/70) ≈ 56.571429 mph, or 56.6 mph. This single observation does not establish factory calibration behavior or legal compliance.
Frequently Asked Questions
Why does my speedometer read higher than my GPS?
Differences can reflect instrument calibration, tire rolling circumference, or uncertainty in the GPS observation. A worn or smaller rolling tire tends to raise the indicated reading relative to actual speed if calibration is unchanged. GPS reception, sampling and changing speed can also affect a comparison. A proper test under the relevant manufacturer or approval procedure is needed to assess accuracy; a small difference is not automatically proof of normal behavior.
Do bigger tires make my speedometer read faster or slower?
If the original calibration was accurate and remains unchanged, a greater rolling circumference increases actual speed for the same indicated reading, so the speedometer reads low relative to this model. A smaller rolling circumference has the opposite effect. Existing display bias or a recalibration can change the final result, so verify the actual reading after a tire change.
Does a tire size change affect the odometer too?
The tire model applies the new/original rolling-size ratio to distance if the odometer was also accurate on the original tires and its calibration is unchanged. Larger rolling size then causes under-recording and smaller size causes over-recording. Signed indicated-versus-actual error uses actual distance as the denominator; it is not the same percentage as positive diameter change. Speedometer display bias need not match odometer bias, and a GPS speed pair alone does not justify an odometer correction. The calculator shows estimated actual distance per 10,000 indicated miles or kilometers.
