# Tyre Size Speed Calculator

Tyre size speed calculator. Compare original and new tyre specs, get actual speed, speedometer error percent, diameter change, and corrected MPH or km/h reading after a tyre upsize.

## What this calculates

Fit a taller tyre and your speedometer starts reading low. Fit a shorter one and it reads high. This tyre size speed calculator takes the original and new tyre specs in standard metric format (width / aspect R wheel, e.g. 225/70R16), works out overall diameters, and returns your actual ground speed, the speedometer error as a percentage, and the corrected MPH or km/h the gauge should be showing.

## Inputs

- **Original Tyre Size** — options: 155/80R13, 175/70R14, 195/65R15, 205/55R16, 215/60R16, 225/70R16, 245/70R16, 235/75R17, 265/70R17, 275/65R18, 285/70R17, 315/70R17 — Factory tyre spec in metric width / aspect R wheel format. Switch mode below for a custom diameter input.
- **New Tyre Size** — options: 155/80R13, 175/70R14, 195/65R15, 205/55R16, 215/60R16, 225/70R16, 245/70R16, 235/75R17, 265/70R17, 275/65R18, 285/70R17, 315/70R17 — New tyre spec you are fitting.
- **Speedometer Reading** (mph) — min 0 — Speed your speedo is currently showing.

## Outputs

- **Original Tyre Diameter** (in) — Overall diameter of the original tyre.
- **New Tyre Diameter** (in) — Overall diameter of the new tyre.
- **Diameter Change** (%) — Percent change in overall tyre diameter.
- **Actual Speed** (mph) — Real ground speed at the indicated reading.
- **Actual Speed** (km/h) — Real ground speed in km/h.
- **Speedometer Error** (%) — Positive = speedo reads slow (actual speed is higher). Negative = speedo reads fast.
- **Target Speedo Reading for True 60 mph** (mph) — What your speedometer should display when you are actually doing 60 mph.

## Details

How the tyre size speed calculator works

A tyre's overall diameter comes from three numbers on the sidewall: section width in millimetres, aspect ratio (sidewall height as a percentage of width), and rim diameter in inches. The formula is:

diameter (inches) = rim diameter + 2 x (width x aspect / 100) / 25.4

For a 225/70R16: sidewall = 225 x 0.70 = 157.5 mm, diameter = 16 + (2 x 157.5) / 25.4 = 16 + 12.40 = 28.40 inches. For a 245/70R16: sidewall = 245 x 0.70 = 171.5 mm, diameter = 16 + (2 x 171.5) / 25.4 = 29.51 inches.

Your speedometer measures driveshaft turns, not ground speed, and converts using the factory tyre diameter. When you fit bigger tyres, each wheel revolution covers more ground but the gauge still assumes the stock distance, so the displayed number drops below the real speed. The speed correction factor is new diameter / original diameter. Multiply the indicated speed by that factor to get actual speed.

Plus-sizing rule of thumb

  
    ChangeDiameter impactSpeedo error (indicated 60)
  
  
    +10 mm width, same aspect+0.55 in (+1.9%)Actual 61.2 mph
    +1 inch rim, lower profile (plus-one)+/- 0.3 inWithin 1% (correct)
    Stock 225/70R16 to 245/70R16+1.11 in (+3.9%)Actual 62.3 mph
    Stock 265/70R17 to 315/70R17+2.75 in (+8.8%)Actual 65.3 mph
  

Staying legal and safe

Most jurisdictions (US, UK, EU, Australia, NZ) allow a speedometer to read up to 10 percent high but never low. After a tyre upsize the gauge shifts toward reading low, so you can quickly end up driving faster than you think. Use this tyre size speed calculator before a fitment change and plan to stay under a 3 percent diameter increase if you do not want to recalibrate. Above 3 percent you should retune the speedometer through the ECU or swap the driven gear on older manual gearboxes.

What to measure if the sidewall is worn off

If your tyre has no readable spec (common on trailers and older vehicles), measure overall diameter with a tape at the centre of the tread, then a second time at 90 degrees to catch any out-of-round. Take the average. You can feed a custom diameter into the plain speedometer calculator, which supports both tyre size and axle-gear inputs. Note that a worn tyre is typically 0.5-1.0 inch smaller than a new one, which adds 2-4 percent to the speedo reading before you even change sizes.

## Frequently Asked Questions

**Q: How does a tyre size speed calculator work?**

A: It parses each tyre's sidewall spec (width / aspect R wheel) into an overall diameter in inches, divides new by original to get the correction factor, and multiplies the indicated speedo reading by that factor. Bigger tyres make the speedo read low, so actual speed is higher than indicated.

**Q: Is fitting bigger tyres legal?**

A: In most countries a speedometer error of up to 10 percent low is outside legal tolerance, so large tyre upsizes without ECU recalibration break the rule. A 225/70R16 to 245/70R16 change pushes the speedo 3.9 percent low, which is borderline. A 265/70R17 to 315/70R17 change is 8.8 percent low, almost at the limit. Check your local regulations before fitting oversize tyres.

**Q: How do I find my tyre diameter from the sidewall code?**

A: Take the width in millimetres, multiply by the aspect ratio (as a decimal) to get the sidewall height, double it because there is a sidewall on top and bottom of the wheel, convert to inches, then add the rim diameter. For 225/70R16: 225 x 0.70 = 157.5 mm sidewall, x 2 / 25.4 = 12.40 in sidewall pair, + 16 = 28.40 in total diameter.

**Q: Does tyre width change the speedometer reading?**

A: Only indirectly. The speedo cares about overall diameter, not section width. But a wider tyre at the same aspect ratio has a taller sidewall (aspect is a percent of width), so it raises the diameter. A 245/70R16 is 1.1 inches taller than a 225/70R16 purely because the 245 section width makes the 70 percent sidewall taller.

**Q: Will a plus-one or plus-two tyre affect my speedo?**

A: A properly-chosen plus-one or plus-two fitment keeps the overall diameter within 1 percent of stock, so the speedometer barely changes. That is the whole point of plus-sizing: bigger rims, lower-profile sidewall, same rolling diameter. Use this tyre size speed calculator to confirm the diameter change is under 1 percent before you order.

**Q: How do I recalibrate my speedometer after new tyres?**

A: On modern OBD-II vehicles, a handheld tuner (SCT, Bully Dog, Hypertech) lets you enter the new tyre size and the ECU adjusts the speedometer constant. Older cable-driven speedometers use a replaceable driven gear in the transmission tail housing with tooth counts chosen to match the ratio change. A few shops offer inline speedo correction boxes like the Yellow Box for late-model vehicles where flashing the ECU is not practical.

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Source: https://vastcalc.com/calculators/physics/tyre-speed
Category: Physics
Last updated: 2026-04-08