Tire Size Calculator

Compare two tire sizes to see dimensional differences, speedometer error percentage, and actual speed at common indicated speeds. Enter sizes like 225/45R17.

Original Tire

Width (mm)

Aspect Ratio (%)

Rim Diameter (inches)

New Tire

Width (mm)

Aspect Ratio (%)

Rim Diameter (inches)

Measurement	Original	New	Difference

Speedometer vs Actual Speed

Speedo shows	Actual speed

—

Speedometer Error

—

Rev/mile difference

What is a Tire Size Calculator?

A tire size calculator decodes the alphanumeric string printed on tire sidewalls — such as 225/45R17 — and computes the physical dimensions of the tire: overall diameter, sidewall height, and circumference. It also calculates the speedometer error and RPM difference that results from changing to a different tire size, which is essential information for anyone upgrading wheel size, fitting wider tires, or installing aftermarket wheels. The three core numbers in a tire size code are the section width in millimetres (225), the aspect ratio (45 — the sidewall height as a percentage of section width), and the rim diameter in inches (17).

Changing tire size affects more than just appearance — it has measurable performance and safety implications. A larger overall diameter increases the wheel's rolling circumference, which means the wheel travels further per revolution than the vehicle's speed sensor expects, causing the speedometer to read lower than the actual speed. Conversely, a smaller overall diameter causes the speedometer to over-read. For safety and legal compliance, the tire size calculator verifies that a proposed size change keeps speedometer error within acceptable limits (typically ±3% in most jurisdictions) and confirms that the new tires will clear wheel arches and suspension components without rubbing.

Plus-sizing — fitting larger diameter wheels with lower-profile tires to maintain the same overall diameter — is a common customisation that preserves speedometer accuracy while improving handling response due to shorter, stiffer sidewalls. The tire size calculator enables plus-sizing by finding combinations of wheel diameter, section width, and aspect ratio that produce approximately the same overall diameter as the stock tire. This calculation is critical when fitting winter tires on a spare set of wheels, comparing tire specifications across brands, or verifying that a performance tire upgrade is dimensionally compatible with the vehicle.

How the Tire Size Calculator Works

Formula, assumptions, and calculation steps for this automotive tool.

Formula Used

Tire Diameter = Rim Diameter (in) + 2 x (Section Width x Aspect Ratio / 100) / 25.4

Methodology

Decodes the tire size code, which is width, aspect ratio, and rim diameter, to compute overall tire diameter and compare sizes for fitment.

Calculation Steps

Enter distance, fuel use, price, payment, or vehicle value assumptions.
Normalize miles/kilometers, gallons/liters, and monthly periods.
Apply the relevant cost, efficiency, or depreciation formula.
Show per-trip, monthly, or ownership totals.

Assumptions and Limits

Fuel prices, insurance, taxes, and resale values change over time.
Driving style and maintenance history affect real costs.
Use results for planning and comparison.

Frequently Asked Questions

How do I read a tire size?

A tire size like 225/45R17 means: 225 = section width in millimetres, 45 = aspect ratio (sidewall height as % of width), R = radial construction, 17 = rim diameter in inches. Sidewall height = width × aspect ratio / 100. For 225/45: sidewall = 225 × 0.45 = 101.25mm. Overall diameter = (2 × sidewall) + (rim diameter × 25.4).

How much speedometer error is acceptable?

Most countries allow ±2.5–4% speedometer tolerance from the factory. A speedometer error of less than 3% from a tire size change is generally considered safe and acceptable. However, larger errors affect odometer readings (affecting warranty mileage and oil change intervals), GPS accuracy comparisons, and can technically cause you to speed without realising it. Errors over 5% should be corrected with a speedometer recalibration.

What is plus-sizing (staggered fitment)?

Plus sizing means increasing the rim diameter while decreasing the sidewall height to keep overall diameter the same (or very close). For example, going from 215/60R16 to 225/50R17 — the rim is 1 inch larger but the shorter sidewall compensates. Benefits include sharper handling and aesthetics. Downsides include less ride comfort and more vulnerability to pothole damage.

Can I fit a wider tire on my car?

Wider tires improve grip and appearance but require sufficient clearance in the wheel well. Generally, you can safely go up 10–20mm in width from the OEM size without rubbing. Always verify clearance at full steering lock and with suspension compressed. Check with a tire fitment guide or ask a tire shop to confirm compatibility for your specific vehicle.

Real-World Applications

🚗

Wheel Upgrade & Plus-Sizing Verification

Drivers upgrading to larger diameter wheels (plus-sizing) — e.g. from 17-inch to 19-inch wheels — use the tire size calculator to find a tire profile that keeps the overall diameter within 2–3% of the original, preserving speedometer accuracy and preventing changes to the gear ratio, ABS calibration, and electronic stability control. The calculator shows the exact overall diameter for both the original and proposed tire, quantifying the delta.

❄️

Winter Tire & Second Set Fitment

Many drivers fit winter tires on a separate set of steel wheels to avoid mounting/dismounting costs. The tire size calculator verifies that the chosen winter tire dimensions — often a narrower section width and same rim diameter for better snow penetration — are compatible with the vehicle's wheel arch clearance and that the speedometer error introduced by any diameter change is within the legally acceptable range (typically ±3%).

🏎️

Performance & Motorsport Tyre Selection

Motorsport competitors using production-based class regulations often have strict rules on allowed tire dimensions. The tire size calculator verifies that aftermarket performance tires (wider section, lower profile) meet overall diameter requirements and confirms clearance from suspension and wheel arch components at full lock and full bump travel. For track day drivers, it also calculates the change in final drive ratio effect from a different rolling circumference.

🚐

Load Capacity & Van / SUV Specification

Fleet managers specifying tires for commercial vans, 4×4s, and light trucks cross-check tire width and load index against the vehicle's payload requirement using the tire size calculator. Correct load rating verification — ensuring the new tire's load index meets or exceeds the manufacturer's minimum — is a legal and safety requirement that the dimensional calculator supplements by confirming that the wider commercial tires clear the vehicle's body at full steering lock.

📊

Speedometer Calibration After Lift Kit

4×4 owners fitting suspension lift kits and larger off-road tires use the tire size calculator to quantify speedometer error before deciding whether to recalibrate the vehicle's ECU or GPS speedometer. A 33-inch mud-terrain tire on a vehicle that came with 29-inch all-terrain tires has a 13.8% larger circumference — the speedometer will under-read actual speed by approximately 12%, a legally significant error in jurisdictions with strict speed camera enforcement.

🔢

Tyre Label & EU Regulation Compliance

EU tyre labelling regulations require retailers to display fuel efficiency rating, wet grip rating, and noise level for each tire. Fleet procurement officers and consumer buyers use tire size calculators alongside these labels to compare dimensionally equivalent tires — same overall diameter, same load index — across brands, evaluating whether a fuel-efficient but slightly noisier tire suits their driving profile better than a quieter option with lower wet grip performance.

Common Mistakes

Confusing the aspect ratio percentage with a millimetre measurement

The aspect ratio (the middle number in a tire size code) is a percentage — 45 means the sidewall height is 45% of the section width, not 45mm. For a 225/45 tire, the sidewall height is 225 × 0.45 = 101.25mm. Treating the aspect ratio as a millimetre measurement and attempting to add it directly to rim diameter in millimetres produces a wildly incorrect overall diameter calculation.

Forgetting to convert rim diameter from inches to millimetres for the overall diameter formula

The standard tire size formula for overall diameter in millimetres is: (Sidewall height mm × 2) + (Rim diameter inches × 25.4). Mixing inches and millimetres without applying the 25.4 conversion factor for the rim diameter produces a result that is approximately 25 times too small. This unit-mixing error is the most common manual calculation mistake in tire sizing.

Ignoring load index and speed rating when changing tire size

The tire size code suffix (e.g. 95V) encodes the load index (95 = 690 kg per tire) and speed rating (V = max 240 km/h). Choosing a dimensionally identical replacement tire with a lower load index can exceed the tire's rated capacity at the vehicle's GVWR, a safety and legal issue. Always verify that the replacement tire's load index equals or exceeds the vehicle manufacturer's minimum specification — not just the overall diameter.

Assuming a ±3% speedometer error is always legally acceptable

The ±3% tolerance is a guideline common in many jurisdictions, but specific rules vary: the UK MOT test permits up to +6%/−0% speedometer error (the speedo may over-read but must not under-read); EU vehicle type approval requires speedometer error to remain within +10%/−0% of actual speed. A tire size change that causes the speedo to significantly under-read actual speed can result in inadvertent speeding. Verify the jurisdiction-specific rules before approving a tire size change.

Not checking wheel well clearance at full lock and full suspension travel

A wider or taller tire may calculate as within acceptable overall diameter tolerance but still physically contact the wheel arch, inner wing, or suspension components at maximum steering lock or full bump travel. The tire size calculator confirms dimensional compatibility in terms of rolling circumference; physical fitment clearance requires consulting vehicle-specific fitment guides, owner forum data, or physical test fitting on the actual vehicle before committing to a full set of tires.

Tire Size Code Decoder Quick Reference

Code Element	Example	Meaning
Section width	225	Tyre width in mm (tread face)
Aspect ratio	45	Sidewall = 45% of section width
Construction	R	R = Radial; B = Bias-belted
Rim diameter	17	Wheel diameter in inches
Load index + speed	95V	95 = 690 kg; V = 240 km/h max