Tire Size Speedometer Variance

Changing tire diameter changes how far the vehicle travels per wheel revolution, and the speedometer keeps counting revolutions as though nothing happened. This works out your actual speed and the percentage error, which also applies to the odometer and every mileage-based figure downstream of it.

How to use it

1. Enter the stock tire diameter in inches.
2. Enter the new tire diameter.
3. Enter an indicated speed to see the actual speed and variance.

Why the error is proportional

A speedometer measures rotational speed at the transmission output or a wheel sensor, then multiplies by a factor derived from the stock tire circumference and the final drive ratio. It has no way of knowing what tire is fitted.

A larger tire covers more ground per revolution, so at any given indicated speed you are actually going faster. The ratio is simply new diameter divided by old.

A worked case: 30 inch stock, 32 inch fitted, is a ratio of 1.0667. At an indicated 65 mph the actual speed is about 69.3 mph — 4.3 mph fast, a 6.67 percent error. Because the relationship is proportional, the absolute error grows with speed: at an indicated 30 mph it is 2 mph, at 80 mph it is 5.3.

Smaller tires reverse this. Fitting a 28 inch tire in place of a 30 gives a ratio of 0.933, so an indicated 65 mph is actually about 60.7 — the speedometer reads high and you are going slower than it says.

Finding the actual diameter

Tire sizes are not stated as diameters, so the number has to be derived from the sidewall marking.

For a metric size such as 265/70R17: the section width is 265 mm, the aspect ratio is 70 percent, so the sidewall height is 185.5 mm. Diameter is the wheel diameter plus twice the sidewall: 17 inches is 431.8 mm, plus 371 mm gives 802.8 mm, or about 31.6 inches.

Flotation sizes such as 33x12.50R15 state the diameter directly — nominally 33 inches, though actual measured diameters typically run half an inch under the nominal figure.

Two caveats. Published specifications are for a specified inflation pressure and no load; a loaded tire has a smaller rolling radius than its free diameter, and this is why measured error sometimes differs slightly from calculated. And tires wear: a tire at the wear bars is roughly half an inch smaller in diameter than new, which is about 1.5 percent and enough to shift a marginal calibration.

Everything downstream is affected

The speedometer is the visible symptom. The same error propagates through several systems.

• The odometer under-records with larger tires, at the same percentage. Six percent over 100,000 miles is 6,000 unrecorded miles, which shifts service intervals, affects warranty coverage, and misstates the figure on a resale listing.
• Fuel economy calculated from odometer miles is wrong by the same percentage, so a real 20 mpg reads as about 18.8 with 6 percent larger tires — often blamed on the tires themselves when part of it is measurement.
• Effective gearing changes. A larger tire raises the overall ratio, which reduces low-speed torque at the wheels and can cause the transmission to hunt between gears. Regearing the differential is the usual remedy on significantly oversized tires.
• Some transmission shift schedules and ABS or stability systems use wheel speed inputs, and a large error can affect calibration.
• Cruise control holds an indicated speed, so the actual speed carries the same error.

Factory bias and recalibration

Most manufacturers deliberately calibrate speedometers to read slightly high, commonly 1 to 3 percent, and regulations in many jurisdictions permit an over-reading while prohibiting an under-reading. So a stock vehicle indicating 65 may genuinely be doing 63.

That bias partially masks a small tire upsize and compounds a downsize. It also means a calculated error should be applied to your actual indicated reading rather than assuming the stock speedometer was accurate to begin with. A GPS reading over a steady stretch is the practical way to establish the real baseline.

Recalibration is straightforward on most modern vehicles: a dealer or a handheld programmer can update the tire size parameter in the powertrain module, correcting speedometer and odometer together. Older vehicles with mechanical cables need a different speedometer gear. Standalone correction devices exist and work, but they adjust the signal rather than the underlying parameter and may not correct every system that uses it.

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