Speedometer error calculator
How fast you really go, and how many miles the odometer misses, after new tires or gears or against a GPS.
A size like 285/70R17 or 33x12.50R15, or the diameter in inches.
Only matters if the speed sensor is on the transmission or driveshaft. Keep both the same if you did not change the gears.
Why the speedometer is wrong after a change
A speedometer does not measure speed. It counts how often something turns and assumes how far the car moves each turn. Change the tire's diameter or the gearing between the pickup and the wheels, and the assumption fails. This page puts a number on it for speed and for the odometer, and then compares the result with the tolerance in the UN regulation for speedometers.
The method
The speedometer counts rotations of something that turns in proportion to road speed (a wheel sensor, the transmission output, or a cable driven from it) and converts them into mph using a built-in assumption about how far the vehicle moves per rotation. If the tire diameter changes, the distance per rotation changes in proportion, but the assumption does not. So real speed = indicated speed × (new diameter ÷ old diameter). If the pickup is after the axle, the axle ratio enters too: real speed = indicated speed × (new diameter ÷ old diameter) × (old axle ÷ new axle). The odometer works the same way, so the percentage error is the same for speed and distance.
Worked example
Changing from 265/70R17 (31.6 inches) to 285/70R17 (32.7 inches), a speedometer showing 70 mph means 72.4 mph, an error of 3.4 percent. The ratio 32.7 ÷ 31.6 = 1.0349 is also the odometer ratio: 1,035 real miles per 1,000 indicated, so a 12,000-mile year shows 11,596 miles on the dash.
Common mistakes
Using the nominal tire diameter from the size code but forgetting that the rolling diameter is smaller than the unloaded diameter by about the same amount on both tires; the ratio of two diameters survives that, but the absolute speed does not, which is why a GPS check is better than a calculation if you need precision. Second, forgetting that the speedometer may already have been recalibrated for the new tires, in which case the old-to-new ratio no longer applies. Third, checking against another vehicle's speedometer, which has its own error; only GPS or a radar sign gives a real reference. Finally, tire wear matters: as tread wears the diameter falls, so the error changes through the life of the tire.
Limits of this method
The ratio of diameters is exact only if the tires are inflated to the same pressure and carry the same load, because tires shrink under load. The calculation assumes a speedometer that is correct today with the old tires; if it was already reading high (as the regulation allows), the new error adds to it. We also read the Regulation No. 39 text in an older revision, from a copy of the Supplement 5 text, and did not open the newer 2018 and 2025 editions, so check the current text before relying on the numbers for an approval. Nothing here is a U.S. legal test; the speedometer rule that matters for a ticket is the speed a police radar or timed measurement shows.
How much does the speedometer change with bigger tires?
In proportion to the tire's diameter. A 285/70R17 is 32.7 inches tall against 31.6 for a 265/70R17, a ratio of 1.0349, so the vehicle goes 3.5 percent faster than the speedometer says: 72.4 mph when it shows 70. The odometer is off by the same factor and counts 1,035 real miles for every 1,000 it shows.
Does changing the axle ratio affect the speedometer?
It does if the speedometer reads from the transmission output or a driveshaft sensor, as many do, because the axle then turns at a different rate for the same road speed. Going from a 3.73 to a 4.10 axle with the same tires, the real speed is 63.7 mph at a reading of 70, which is 9.9 percent high on the display. If the sensor sits at the wheels (as with the anti-lock brake speed sensors that some vehicles use), the axle change has no effect, and only the tire diameter matters. Enter the same ratio in both fields to ignore it.
How do I check my speedometer against GPS?
Drive at a steady speed on a flat road and compare the display with a GPS speed reading, which is based on measured position change and is generally more accurate than a vehicle speedometer. If the display says 65 mph and the GPS says 62, the display is 4.8 percent high. Enter the two readings in the second mode and the calculator returns the correction factor 1.0484. Several steady runs at different speeds help, because an error that comes from tire size scales with speed while an offset does not.
Is it legal for a speedometer to read high?
UN Regulation No. 39, in the revision we read, requires that the indicated speed is never below the true speed, and at the test speeds that it exceeds the true speed by no more than 0.1 × true speed + 4 km/h. At a true 100 km/h the display may therefore show 100 to 114 km/h: a reading of 0, 14 and 15 km/h above the truth is allowed, allowed and rejected respectively. For production vehicles checked in the same regulation's conformity annex, cars and light trucks get +6 km/h instead of +4. We did not find an equivalent U.S. federal limit, so the page does not state one.
Why does my dash read higher than my GPS?
The regulation described above forbids a display that reads low and tolerates one that reads high, so a gap in the dash's favor is normal and within the rule. At 65.0 mph (104.6 km/h) that tolerance band would be up to about 14 km/h above the true speed at type approval. Tire wear, inflation and a different tire size from the factory one move the figure too.
How do I fix the speedometer after changing tires?
Use a recalibration: a dealer or a shop can reprogram the electronic speedometer, and bolt-on programmers, calibration modules or a different speedometer gear (on older mechanical drives) can correct it. The calculator gives the correction factor: multiply the old calibration by it. Check the result against GPS afterward, because the odometer, cruise control and transmission shift points usually follow the same signal.
Sources: UN Regulation No. 39, Revision 1 (document E/ECE/324 Rev.1/Add.38/Rev.1, text up to Supplement 5, in force 7 December 2002): paragraph 5.2.5 test speeds, the 0 ≤ V1 − V2 ≤ 0.1 V2 + 4 km/h relationship, and Annex 3, +6 km/h for production checks of categories M and N. We read a copy of this text, not the UNECE original or the newer editions. The ratios of tire diameters and axle ratios are arithmetic. Related: tire size calculator, axle ratio change calculator, RPM to MPH calculator. How we check our calculators.