Tire Size and Speedometer Error
Bigger tires, slower speedometer. Here is exactly how much.
Speedometer error equals the diameter change percentage: new diameter divided by old, minus one. Moving from 265/70R17 to 285/70R17 is a 3.5 percent increase, so an indicated 60 mph is really 62.1 mph and the odometer logs only 96.6 miles per 100 traveled. Stay within 3 percent of stock to keep error negligible; beyond that, recalibrate with a tuner or dealer tool.
The error formula
True speed = indicated speed x (new diameter / old diameter). With 32.7 / 31.6 = 1.035, every indicated speed is 3.5 percent low: 30 reads as 31.0, 60 as 62.1, 70 as 72.4.
The odometer errs the same way but inverted: it counts revolutions, so it logs fewer miles than traveled, about 96.6 per 100 in our example.
Why it matters
Speeding tickets are the obvious risk: 62 in a 60 zone because the gauge said 60. Odometer under-counting also skews fuel economy math and can complicate lease mileage and warranty tracking.
ABS and traction control tolerate small changes but are calibrated to stock revs per mile; large deviations can confuse them, another reason for the 3 percent rule.
Fixing it
Within 3 percent, most drivers just learn the offset. Beyond that, recalibration options include handheld tuners, dealer reprogramming, and aftermarket speedometer calibrators that intercept the signal.
Recalibration usually requires the new revs per mile, which the calculator gives you: about 638 stock versus about 616 for the 285 in our example (63,360 / 102.7).
Skip the arithmetic
See your speedometer error on the calculator.
Speedometer error questions
How much will bigger tires throw off my speedometer?
Multiply indicated speed by new diameter over old diameter. A 3.5 percent diameter increase, like 265/70R17 to 285/70R17, makes an indicated 60 mph actually 62.1 mph.
Does the odometer change too?
The odometer counts revolutions, so larger tires log fewer miles than traveled: about 96.6 miles per 100 actual miles with a 3.5 percent diameter increase. This flatters MPG math and affects service intervals.
Can I recalibrate my speedometer for bigger tires?
Most modern vehicles can be recalibrated by updating the programmed revs per mile or tire size. Handheld tuners, dealer tools, and inline calibrators all do the job; you will need the new tire's revs per mile.