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Walk-through

How to Use This Calculator

3 steps

1

Enter your stock and new tire diameters

Type the stock (original-equipment) tire's overall diameter and the diameter of the tire you actually have mounted, both in inches. If you don't know the diameters, switch to "Tire size code" mode and enter the size printed on the sidewall — e.g. 225/60R16 — instead.

2

Enter your speedometer reading

Type the speed your speedometer currently shows (or a speed you commonly drive at). The calculator instantly converts that indicated speed into your true, actual road speed based on the ratio between the two tire diameters.

3

Check the % Error and Odometer tabs

The True Speed tab shows your corrected speed. The % Error tab shows the same error at other common speeds (20-80 mph). The Odometer tab shows how the same diameter change throws off your trip mileage and odometer reading.

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Reference

Formula & Methodology

3 formulas

True speed from a diameter ratio

true speed = indicated speed × (new diameter ÷ stock diameter)

A speedometer is calibrated against the tire diameter (and resulting rolling circumference) the vehicle shipped with. Because road speed is proportional to how far the tire rolls per wheel revolution, fitting a larger-diameter tire makes each revolution cover more ground than the speedometer assumes — so the true speed is higher than what's shown. A smaller tire has the opposite effect. Circumference scales linearly with diameter (circumference = π × diameter), so the diameter ratio alone gives the correction factor without needing tire width or aspect ratio.

Percent speedometer error

% error = (true speed − indicated speed) ÷ indicated speed × 100

Expresses the same diameter-ratio correction as a percentage. A positive % error means the true speed is higher than shown (you're going faster than you think); negative means the true speed is lower (you're going slower than shown).

Odometer error

odometer ratio = stock diameter ÷ new diameter

The odometer works the same way as the speedometer: it counts wheel revolutions and multiplies by the stock tire's assumed circumference. With a larger new tire, each revolution covers more real distance than the odometer credits it for, so the odometer under-reads the miles actually driven. With a smaller new tire, the odometer over-reads.

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Glossary

Key Terms Explained

6 terms

Speedometer error The gap between the speed a vehicle's speedometer displays and the vehicle's true road speed, caused by a mismatch between the tire diameter the speedometer was calibrated for and the tire diameter actually mounted.

Tire diameter The overall (outside) diameter of an inflated, mounted tire, measured in inches — rim diameter plus twice the sidewall height. This is the value that determines how far the vehicle travels per wheel revolution, not the tire's width or aspect ratio alone.

Rolling circumference The distance a tire covers in one full revolution, equal to π × the tire's overall diameter. Speedometers and odometers both derive their readings from an assumed rolling circumference.

Odometer The instrument that tracks total (and trip) distance traveled by counting wheel or transmission-output revolutions and multiplying by the stock tire's rolling circumference. It drifts out of calibration for the same reason the speedometer does whenever tire diameter changes.

True speed The vehicle's actual road speed, corrected for the real tire diameter — as opposed to the indicated speed shown on an uncalibrated speedometer.

Gear ratio The ratio between a drivetrain's input and output rotational speeds (e.g. transmission gear or final-drive ratio). Gear ratio and tire diameter both feed into the same speed/RPM relationship — a diameter change affects real-world speed exactly as if the final-drive ratio had shifted by the same percentage.

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Scenarios

Real-World Examples

3 worked examples

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Off-road upgrade

26 in stock tire swapped for a 28 in tire

Stock diameter 26 in New diameter 28 in Speedometer reading 60 mph

The diameter ratio is 28 ÷ 26 = 1.0769, so true speed = 60 × 1.0769 ≈ 64.6 mph — about 7.7% faster than the speedometer shows. At highway speeds this is enough to accidentally exceed a limit while the dash reads well under it.

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Smaller spare or worn-down tire

26 in stock tire replaced with a 24 in tire

Stock diameter 26 in New diameter 24 in Speedometer reading 60 mph

The ratio is 24 ÷ 26 = 0.9231, so true speed = 60 × 0.9231 ≈ 55.4 mph — the speedometer over-reads by about 7.7%. You'd actually be driving slower than the dash claims, and every mile shown on the odometer represents fewer real miles driven, inflating apparent fuel economy.

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Trip-mileage impact

Same 26 in → 28 in tire, tracking odometer drift

Stock diameter 26 in New diameter 28 in

The odometer ratio is 26 ÷ 28 = 0.9286, so for every 100 miles actually driven the odometer shows only about 92.9 miles — the odometer under-reads by roughly 7.1%. Over a long road trip, mileage-based maintenance schedules and fuel-economy calculations will look better than reality.

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Deep Dive

Understanding the Speedometer Error Calculator

3 chapters · 4 min read

Changing your tire size — for looks, off-road clearance, or simply because a different size was available — changes how far your vehicle actually travels per wheel revolution. Because the speedometer and odometer are both calibrated for the tire diameter your vehicle shipped with, any diameter change throws both readings off by the same percentage. This calculator converts an indicated speedometer reading into your true road speed, shows the percentage error at common speeds, and quantifies how far your odometer's mileage drifts from reality.

How the Speedometer Error Calculator works

Vehicle speed sensors measure wheel (or transmission output shaft) rotational speed and convert it to a road speed using the rolling circumference of the tire the factory installed. Rolling circumference is simply π times the tire's overall diameter, so it scales linearly with diameter — a tire 7.7% larger in diameter covers 7.7% more distance per revolution. The calculator applies that same ratio: true speed equals the indicated speed multiplied by (new diameter ÷ stock diameter). The odometer uses the identical revolution count and the identical stock-circumference assumption, so it drifts by the inverse of that ratio (stock diameter ÷ new diameter) in the opposite direction of the speedometer's error.

Inputs and what they mean

Enter the stock and new tire diameters in inches, or switch to tire-size-code mode and enter the size printed on the sidewall (e.g. 225/60R16 or a flotation size like 33X12.50R15) — the calculator derives the overall diameter from the width, aspect ratio, and rim size automatically. The speedometer reading is whatever your dashboard currently shows; the calculator instantly recalculates the true speed, percent error, and odometer impact as you type. Diameter, not width or load rating, is what drives the error — two tires with very different widths but the same overall diameter produce no speedometer error at all.

Limits and edge cases

This calculator assumes a purely geometric relationship between tire diameter and speed, which is the same assumption speedometers themselves are built on. It does not account for tire pressure, load, tread wear, or speed-dependent flex, all of which cause small (typically under 1-2%) additional variance in real rolling circumference beyond the nominal size. It also doesn't apply to vehicles whose speedometer has been electronically recalibrated after a tire change — some newer vehicles support this through the instrument cluster or a dealer tool, which would make the indicated and true speeds match again regardless of tire size. If your factory speedometer is already off by a known amount from a separate calibration check, treat this calculator's output as an estimate layered on top of that baseline error, not a replacement for a GPS speed check.

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Questions

Frequently Asked Questions

6 questions

If I put bigger tires on my car, will my speedometer read too high or too low?+

Too low. A larger-diameter tire covers more ground per wheel revolution than the speedometer's stock calibration assumes, so the speedometer under-reports your actual speed — you're going faster than the dash shows. The odometer, by the same logic, under-reports distance traveled.

What's the formula this calculator uses?+

True speed equals the indicated speedometer reading multiplied by the ratio of the new tire's diameter to the stock tire's diameter: true speed = indicated × (new Ø ÷ stock Ø). Percent error is (true speed − indicated) ÷ indicated × 100. The odometer error uses the inverse ratio, stock Ø ÷ new Ø, since a bigger tire means fewer wheel revolutions — and therefore fewer odometer counts — per mile actually driven.

Does the odometer read high or low with a larger tire?+

Low. The odometer counts wheel revolutions and multiplies by the stock tire's circumference. A larger tire needs fewer revolutions to cover the same distance, so the odometer racks up fewer counted miles than you actually drove — it under-reads. A smaller tire has the opposite effect and over-reads.

Can I enter a tire size code like 225/60R16 instead of a diameter in inches?+

Yes. Switch the input mode to "Tire size code" and enter the size exactly as printed on the sidewall, in either standard metric format (e.g. 225/60R16) or flotation format (e.g. 33X12.50R15). The calculator computes the overall diameter from the width, aspect ratio, and rim size before running the speed and odometer math.

Can I get my speedometer recalibrated after a tire size change?+

On some newer vehicles, yes — certain instrument clusters or drivetrain control modules support a tire-size recalibration through a dealer scan tool or, on some trucks, a driver-accessible menu. Many vehicles, especially older ones, have no such option, in which case an aftermarket speedometer recalibrator (a small inline device on the speed sensor signal) or a re-geared speedometer drive is the usual fix.

What units does this calculator use?+

Diameters are entered in inches and speeds in miles per hour. Tire size codes use the standard sidewall notation (millimeters for width, a percentage for aspect ratio, and inches for rim diameter), which the calculator converts to inches internally — you never need to do that conversion by hand.

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Speedometer Error Calculator

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