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Drivetrain

Speedometer Correction Calculator

Calculate speedometer error after changing axle gear ratio, tire size, or both. Shows true speed vs indicated and correction multiplier using revs per mile and axle ratio.

What this calculator is for

Changing axle gear ratio or tire diameter changes wheel revolutions per mile — the speedometer reads wrong unless you recalibrate the PCM, swap drive gears, or use a correction module.

This calculator shows true speed vs indicated and error percent after a gear swap (3.15 → 3.73) or tire upsize (265/70R17 to 33s).

A good outcome: you know you are actually doing 68 when the needle says 60 before you argue with a radar ticket or over-shift on the highway.

Calculator

How to use this calculator

  1. Enter stock and new axle ratios (e.g. 3.15 → 3.73).
  2. Enter tire sizes as metric (275/70R17) or overall diameters in inches.
  3. Positive error % means the speedometer reads faster than your true speed.

Enter tire sizes as metric (275/70R17) or measured overall diameter in inches.

Positive error % means the speedometer reads high (common with taller tires or shorter gearing that raises revs/mile).

ABS and transmission shift points also use wheel speed — fix speedo for more than just the gauge.

The math: do it without a calculator

Revs/mile = 63,360 ÷ (π × tire diameter in inches)

True speed = Indicated × (old gear × old revs/mi) ÷ (new gear × new revs/mi)

Revs/mile = 63,360 ÷ (π × tire diameter). True = indicated × (old gear × old revs/mi) ÷ (new gear × new revs/mi).

Factory speedo may already have error — compare to GPS before and after changes to validate baseline.

Real-world examples

3.15 → 3.73 with same 26.5″ tire

Higher numeric axle ratio increases revs per mile — speedometer reads about 18% fast (indicated 60 ≈ true 51) if nothing else changed. Many owners see ~+15–20% error on gear-only swaps; verify with GPS.

265/70R17 to 285/70R17 on a half-ton

Taller tire lowers revs per mile — speedo reads slow (true speed higher than indicated). Combine with a gear swap and enter both changes so you do not correct twice in opposite directions.

Troubleshooting & fine-tuning your setup

Speedo Fixed in Math but Still Wrong on the Dash

PCM calibration, mechanical speedo gears, and aftermarket clusters each need different correction methods — this tool only gives the ratio. Tire wear changes diameter over time.

GPS compares true ground speed; differential tire pressures left vs right can add small error.

Frequently asked questions

Speedometer Correction FAQs

Does a gear swap always make the speedo read fast?

A numerically higher axle ratio (e.g. 3.15 → 3.73) usually increases revs per mile — speedometer reads faster than true speed if uncorrected.

How do I fix speedometer error after bigger tires?

Recalibrate with FORScan/PCM tool, change speedo gear (older trucks), or use a correction box — use this calculator for the ratio first.

Will speedo error affect odometer?

Yes — same revs-per-mile relationship drives odometer and trip unless separately corrected.

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