Section width, aspect ratio, and rim diameter determine a tire’s overall size. A Tire Size Calculator applies these values to get diameter, circumference, and revolutions per mile.
New (Replacement) Tire – Click to Compare
Advanced Analysis – Gearing, Load Rating & Rolling Accuracy
A tire’s nominal diameter is measured unloaded. Under vehicle weight and normal inflation, passenger tires typically deflect about 3–5% (this tool uses a 4% estimate). Enabling this adjusts circumference and revolution figures only — nominal dimensions, fitment comparisons, and speed error stay based on the true spec size.
| Indicated (mph) | Actual (mph) | Indicated (km/h) | Actual (km/h) |
|---|
Your speedometer is calibrated to the original tire. When the overall diameter changes, indicated speed no longer matches true road speed.
Tire Size Calculator: Overall Diameter, Circumference, and Speedometer Accuracy for Any Wheel Fitment
This tool converts a tire’s section width, aspect ratio, and rim diameter into overall diameter, circumference, revolutions per mile, and — when comparing two sizes — the resulting speedometer error and gear ratio impact. Drivers checking a replacement tire, off-roaders sizing up before re-gearing, and anyone decoding a load index or speed rating from the sidewall use it to see the real dimensional change before buying.
Entering Section Width, Aspect Ratio, and Rim Diameter
Enter section width in millimeters, aspect ratio as a whole-number percentage, and rim diameter in inches — the same order printed on the sidewall (e.g., 265/70R17). Expand “New (Replacement) Tire” to compare a second size, or “Advanced Analysis” for gear ratio, load index, and speed rating. The formula assumes millimeter width/sidewall and inch rim diameter, the mixed-unit convention used on every tire sidewall worldwide.
How Overall Tire Diameter, Circumference, and Revolutions Are Calculated
Tire section width, aspect ratio, and rim diameter follow the P-metric/Euro-metric sizing geometry set by the Tire and Rim Association (TRA) in the U.S. and ETRTO (European Tyre and Rim Technical Organisation) in Europe. Sidewall height is the aspect ratio applied to section width:
$$\text{Sidewall Height (mm)} = \text{Width (mm)} \times \frac{\text{Aspect Ratio}}{100}$$
Overall diameter adds two sidewalls — top and bottom — to the rim diameter, converting the metric sidewall figure to inches:
$$\text{Overall Diameter (in)} = \text{Rim Diameter (in)} + \left(2 \times \frac{\text{Sidewall Height (mm)}}{25.4}\right)$$
Circumference and revolutions per mile follow directly: $\text{Circumference (in)} = \pi \times \text{Diameter (in)}$, and $\text{Revolutions per Mile} = 63{,}360 \div \text{Circumference (in)}$, using 63,360 as the fixed number of inches in a mile.
The most common input mistake is entering aspect ratio as a decimal (0.70) instead of a whole-number percentage (70) — this collapses the sidewall height to roughly 1% of its real value and returns a diameter barely larger than the bare rim.
The optional “real-world rolling diameter” adjustment applies a 4% reduction, the middle of the typical 3–5% deflection passenger tires show under normal load and inflation. This range is a widely used approximation documented in tire and vehicle-dynamics engineering references on loaded/rolling radius — not a fixed regulatory figure — so it’s applied to circumference and revolution outputs only, while nominal dimensions and fitment comparisons stay on the true spec size.
When comparing two sizes, the tool flags any overall-diameter change beyond ±3%. That threshold is a widely used tire-industry plus-sizing and fitment convention — documented across tire retailer plus-sizing methodology and fitment guides, not a DOT, SAE, or ETRTO specification limit.
Staying inside it keeps speedometer, odometer, and ABS/traction-control readings close to accurate; exceeding it doesn’t guarantee a problem, but it falls outside the range those guides treat as safely predictable.
Section widths below 80mm or aspect ratios below 10% fall outside any real production passenger or light-truck tire and are blocked by this calculator — the P-metric formula doesn’t break mathematically at those values, it just stops describing a tire that’s actually manufactured.
At the other extreme, aspect ratios above roughly 80–85% and rim diameters below 13 inches are mathematically valid but essentially unused on modern road vehicles (mostly found on vintage, agricultural, or specialty fitments); the calculator will still return a result, but treat it as a geometry check rather than confirmation that a matching tire exists.
Standard Load Index and Speed Rating Reference (ETRTO/TRA)
Both tables follow the ETRTO/TRA load index and speed symbol system used across the tire industry worldwide; the full passenger/light-truck load index range runs from 70 to 126.
A P-metric tire’s rated capacity is derated by about 10% when mounted on a light truck or SUV under TRA tables, while an equivalent Euro-metric (no letter prefix) tire of the same load index number isn’t — so matching load index numbers between a P-metric and a Euro-metric replacement doesn’t always mean matching real-world capacity.
These figures assume correct inflation and a tire in good condition; actual load-carrying and stopping performance in service also depend on inflation pressure, tread wear, and road surface.
| Load Index | Max Load (lb) | Max Load (kg) |
|---|---|---|
| 85 | 1,135 | 515 |
| 91 | 1,356 | 615 |
| 97 | 1,609 | 730 |
| 100 | 1,764 | 800 |
| 102 | 1,874 | 850 |
| Speed Rating | Max Speed (mph) | Max Speed (km/h) |
|---|---|---|
| Q | 99 | 160 |
| S | 112 | 180 |
| T | 118 | 190 |
| H | 130 | 210 |
| V | 149 | 240 |
| W | 168 | 270 |
| Y | 186 | 300 |
Common Questions About Comparing Tire Sizes and Fitment
Does a higher load index number always mean a stronger tire?
Yes — load index is ordinal, so a higher number always means more capacity per tire under ETRTO/TRA tables. But a P-metric tire’s rating is derated about 10% on light trucks and SUVs while an equivalent Euro-metric tire isn’t, so matching numbers don’t always mean matching real-world capacity.
What’s the difference between nominal diameter and rolling diameter?
Nominal diameter is the unloaded, as-designed size calculated from the sidewall code. Rolling diameter is smaller, since vehicle weight flattens the contact patch. Enable the rolling diameter option to see circumference and revolution figures adjusted for this typical 3–5% deflection.
What input mistakes most often throw off the results?
Entering aspect ratio as a decimal instead of a whole-number percentage, swapping the width and rim diameter fields, and mixing millimeter and inch values in the same field are the three most common entry errors — each returns a diameter far outside any real tire’s dimensions.
Why does changing tire size affect my speedometer?
Speedometers calculate speed from wheel rotation rate, calibrated to the original tire’s circumference. A larger tire covers more distance per rotation, so the speedometer under-reads; a smaller tire makes it over-read. The speed error table shows this at common speeds.
Do I need new gears after switching to larger tires?
Not necessarily — larger tires alone just lower effective gearing, dropping RPM at a given speed. Re-gearing to a numerically higher axle ratio restores the original RPM. The gear ratio recommendation uses the drivetrain-tuning convention RPM = (MPH × Gear Ratio × 336.13) ÷ Tire Diameter to estimate a target ratio.