Tire Size Comparison: How to Compare Any Two Tyre Sizes and Read the Difference

A tyre size comparison answers one question: how different are these two sizes, and does the difference matter? The comparison starts with the size codes themselves. Take 205/55R16 and 225/45R17 as the worked example on this page. The first is 205 mm wide, with a sidewall that is 55% of the width, on a 16-inch rim. The second is 225 mm wide, with a 45% aspect ratio, on a 17-inch rim. Run the numbers and the first measures 631.9 mm in overall diameter while the second measures 634.3 mm — a difference of +2.4 mm, or +0.4%. That percentage is the figure the rest of the comparison hangs on, because it decides the speedometer error, the gearing change and whether the swap stays inside the commonly used plus-or-minus 3% guideline.

The reason the percentage matters more than the raw millimetres is that the car's instruments count wheel revolutions. A tyre with a larger circumference covers more ground per revolution, so at the same wheel speed the car is moving faster than the speedometer reports. A +0.4% diameter change means a +0.4% speedometer error, and the same percentage shows up in the odometer reading. That is why a comparison that only lists the two diameters is only half done: the useful version also works out the speedometer effect at real road speeds, the sidewall change, the width change and the clearance checks, which is exactly what the individual comparison pages on this site do for each pair.

This is a calculation, not a safety approval. Always check your vehicle's door-jamb placard, load index and speed rating, and confirm fitment with a tyre professional before fitting a different tyre size.

tire size comparison: How to compare any two tyre sizes

Start by decoding both size codes. The first number is the section width in millimetres. The second is the aspect ratio, the sidewall height as a percentage of the width. The final number after the R is the rim diameter in inches. The sidewall height is width times aspect ratio divided by 100, and the overall diameter is the rim diameter in millimetres plus twice the sidewall. For 205/55R16 that is 16 x 25.4 = 406.4 mm of rim plus 2 x 112.8 mm of sidewall, giving 631.9 mm. For 225/45R17 it is 17 x 25.4 = 431.8 mm of rim plus 2 x 101.2 mm of sidewall, giving 634.3 mm. Once you can do that sum for both sizes, the rest of the comparison is arithmetic: subtract the diameters, divide by the original diameter and multiply by 100 to get the percentage difference.

Next, work out the circumference and the revs per mile for each size. The circumference is pi times the diameter, and the revs per mile is a mile divided by the circumference. The 205/55R16 covers 1985.2 mm per revolution and turns 810.7 times per mile; the 225/45R17 covers 1992.7 mm per revolution and turns 807.6 times per mile. Those two figures describe the same change from different angles: the road sees the circumference, the car's sensors count the revolutions. When the revs per mile fall, the gearing effectively lengthens — the engine turns slightly slower at a given road speed — and when they rise, the gearing effectively shortens.

Then check the sidewall and the width separately from the diameter. The sidewall decides ride comfort and steering response: the 225/45R17 has 101.2 mm of sidewall against 112.8 mm for the 205/55R16, so it is the sharper, less cushioned of the two. The width decides grip and rolling resistance: 225 mm against 205 mm means a broader contact patch, which generally helps dry grip and can cost a little in fuel use through higher rolling resistance. Neither of those effects is something a diameter percentage can tell you, which is why a proper comparison looks at all four measurements rather than one.

Worked example: 205/55R16 vs 225/45R17

Measurement 205/55R16 225/45R17 Difference
Overall diameter 631.9 mm (24.9 in) 634.3 mm (25.0 in) +2.4 mm (+0.4%)
Sidewall height 112.8 mm 101.2 mm -11.6 mm
Circumference 1985.2 mm 1992.7 mm +7.5 mm
Revolutions per mile 810.7 807.6 -3.1
Section width 205 mm 225 mm +20 mm
Rim diameter 16 in 17 in +1 in

The diameter difference of +0.4% sits inside the commonly used plus-or-minus 3% guideline, so this is a swap that keeps the speedometer error small. The speedometer table makes that concrete: fitting the 225/45R17 where a 205/55R16 was, an indicated 30 mph is really 30.1 mph, an indicated 50 mph is really 50.2 mph, and an indicated 70 mph is really 70.3 mph. Going the other way, 70 mph indicated becomes 69.7 mph. The odometer drifts by the same +0.4% in each direction, so recorded mileage shifts too. This pair is a classic plus-sizing example: wider, lower-profile, one inch larger on the rim, and almost the same overall diameter — which is precisely the point of plus-sizing.

The plus-or-minus 3% guideline

The guideline says a replacement tyre within plus or minus 3% of the original diameter is generally treated as acceptable from a speedometer point of view. It is a guideline, not a safety approval, and it only covers the diameter. A tyre can be inside 3% on diameter and still be wrong for the car: too wide for the arches, the wrong load index, the wrong speed rating, or the wrong rim width. Treat the 3% figure as the first filter, not the last word. If the difference is larger than 3%, the speedometer error grows in proportion, the gearing shifts by the same percentage, and recalibration becomes worth discussing with a specialist rather than ignoring. On some cars the driver-assistance systems also assume a standard rolling circumference, so a large change can affect more than the speedometer.

What a comparison cannot tell you

A diameter table cannot confirm that a tyre fits your car. Clearance depends on your wheel arches, suspension components and steering geometry, and the only way to check it is physically: turn the steering to full lock in both directions, check the gaps with the suspension compressed, and check again with the car loaded as it will actually be used. The load index and speed rating also need checking against the door-jamb placard, and the rim width must suit the tyre width. Two tyres with the same size code can also differ slightly in real dimensions by brand, model, wear and inflation pressure, because the published figures are nominal new-tyre dimensions. Use the comparison to narrow the choice down to a shortlist, then confirm the shortlist on the car itself — ideally with a tyre professional.

Common mistakes in tyre size comparisons

The most common mistake is comparing only the width and forgetting the aspect ratio, so a wider tyre quietly becomes a taller tyre too. The second is assuming the tyre currently fitted is the correct size, instead of checking the door-jamb placard. The third is treating the diameter percentage as a fitment approval — it is a speedometer guideline, and clearance, load index and speed rating are separate checks. The fourth is mixing sizes across an axle, which the car was not designed for. The fifth is forgetting the spare, the tyre-pressure monitoring system and the speedometer, all of which assume the standard size. Avoid those five and a size comparison does what it is supposed to do: give you the numbers to make a grounded decision.

Popular size comparisons

Frequently asked questions

How do I compare two tyre sizes?

Decode both size codes into diameter, sidewall, circumference and revs per mile, then subtract to find the differences. The diameter difference as a percentage of the original size is the key figure: it decides the speedometer error and whether the swap stays inside the commonly used plus-or-minus 3% guideline. Then check the sidewall and width changes separately, and confirm clearance, load index and speed rating on the car.

What is the plus-or-minus 3% rule in tyre size comparisons?

It is the commonly used guideline that a replacement tyre within plus or minus 3% of the original diameter keeps the speedometer error small. It is a guideline about the speedometer and gearing, not a safety approval: clearance, load index, speed rating and rim width still need their own checks, and a tyre inside 3% on diameter can still rub if it is much wider.

How do I work out the speedometer error from a tyre size change?

Divide the new diameter by the old diameter and multiply the indicated speed by that ratio. For example, fitting a 225/45R17 (634.3 mm) in place of a 205/55R16 (631.9 mm) multiplies every reading by 1.004, so 70 mph indicated becomes 70.3 mph in reality. The odometer drifts by the same percentage.

Does a wider tyre always mean a taller tyre?

No, but it often works out that way if the aspect ratio stays the same, because the sidewall is a percentage of the width. Going from 205/55 to 225/55 would make the tyre taller as well as wider. Plus-sizing avoids that by dropping the aspect ratio as the width rises — 205/55R16 to 225/45R17 keeps the diameter within +0.4%, which is why that pair is used as the textbook example.

What should I check before fitting a different tyre size?

Check the door-jamb placard for the specified size, load index and speed rating. Check the diameter difference against the plus-or-minus 3% guideline. Check physical clearance at full steering lock, with the suspension compressed and the car loaded. Check the rim width suits the tyre, and check the tyre-pressure monitoring system still works after the change. A tyre professional can confirm all of it.