Tire Size Comparison and Plus Sizing: Reading the Numbers on the Sidewall

You’re looking at a set of wheels in a different size than your car came with, or a tire shop quoted you “a slightly bigger tire,” and now you need to know whether it actually fits — and whether your speedometer will still tell the truth. The string on the sidewall, something like 225/45R17, holds all the answers, but only if you know how to turn those three numbers into a real-world diameter. This guide explains what each number means, how two sizes really compare, and the plus-sizing rule that lets you change wheels without breaking your speedo or rubbing the fender.

What the sidewall code means

A P-metric tire size has three parts — written width / aspect R rim:

• Section width (the first number) is the tire’s width in millimetres, measured across the widest part of the tread. In 225/45R17 that’s 225 mm.
• Aspect ratio (the second number) is the sidewall height as a percentage of the width — not an absolute measurement. A 45 means the sidewall is 45% of 225 mm.
• Rim diameter (after the R, for radial) is the wheel diameter in inches — 17 here. Yes, the format mixes millimetres and inches in a single code; that’s just historical convention.

So the one calculation that unlocks everything is the sidewall height:

sidewall (mm) = section width × aspect ÷ 100

For 225/45R17: 225 × 45 ÷ 100 = 101.25 mm of sidewall.

From the code to an overall diameter

The number that actually determines fit, gearing and speedometer accuracy is the overall diameter — the full height of the mounted tire. The sidewall sits above and below the wheel, so:

overall diameter (mm) = (2 × sidewall) + (rim × 25.4)

The 25.4 converts the rim’s inches to millimetres. For 225/45R17: 2 × 101.25 + 17 × 25.4 = 202.5 + 431.8 = 634.3 mm (about 24.97 in).

From the diameter, two more useful figures follow directly:

• Circumference = π × diameter — how far the tire travels per revolution.
• Revolutions per mile = 1,609,344 ÷ circumference (in mm) — how many times the wheel turns per mile, which is what your speedometer and odometer actually count.

The Tire Size Comparison calculator does all of this for two sizes at once and lays the results side by side, so you don’t have to run the arithmetic by hand.

Comparing two sizes: the three numbers that matter

When you put two sizes next to each other, three differences decide whether the swap is sensible:

1. Overall diameter difference. This is the headline figure. A tire that’s taller raises the car and effectively makes the gearing taller; a shorter one does the opposite. The widely used guideline is to keep the overall diameter within about ±3% of the original. Beyond that, your speedometer error grows, the ride height changes noticeably, and on some cars the ABS and traction systems — which infer speed from wheel rotation — can be thrown off.

2. Speedometer error. Your speedo is calibrated for the original tire’s revolutions per mile, so a change in diameter shifts every reading by that same ratio — a tire 3% larger turns an indicated 60 mph into a real 61.8 mph. The comparison tool reports this directly: enter an indicated speed and it gives the true speed on the second size. For the ratio itself, which way the error runs, and how it quietly drags your odometer and fuel-economy figures along with it, see how new tire sizes throw off your speedometer.

3. Clearance. Width and diameter both eat into clearance. A larger diameter adds height — and because the tire grows symmetrically, the radius changes by half the diameter difference, so a 19 mm diameter increase is only about 9.5 mm of extra room needed at the top of the arch. Extra section width pushes toward the suspension on the inside and the fender on the outside. Always check both at full lock and full compression.

Plus sizing without changing the rolling diameter

“Plus sizing” means fitting a larger-diameter wheel while keeping the overall tire diameter roughly the same — the look of a bigger rim and lower-profile tire, without messing up the gearing or speedometer. The trick is to trade sidewall for rim: as you go up an inch in wheel size, you drop the aspect ratio (and often add a little width) so the overall diameter stays put.

A textbook plus-one: 205/55R16 → 225/45R17.

• 205/55R16 overall diameter ≈ 631.9 mm
• 225/45R17 overall diameter ≈ 634.3 mm
• Difference: about +2.4 mm, or +0.38% — well inside the ±3% window.

You get a wider contact patch and a shorter, stiffer sidewall (sharper steering response) while the speedometer stays honest. The cost is a firmer ride and more vulnerability to pothole damage, since there’s less sidewall to absorb impacts. Run any candidate pairing through the Tire Size Comparison calculator and aim to keep the diameter delta near zero.

A few practical cautions

• Load and speed ratings. Comparing geometry is only half the job. A replacement tire must meet or exceed the original’s load index and speed rating — never downgrade them.
• Width vs. rim width. Each tire width has a recommended range of rim widths. A 245-section tire on too narrow a wheel will balloon and handle poorly.
• Staggered fitments. Many performance cars run different sizes front and rear by design. Compare each axle to its own original size, not front-to-rear.
• Mixing the metric and inch parts is the most common arithmetic slip. If you ever want to sanity-check a sidewall figure in inches, the Custom Unit Converter handles the mm-to-inch step cleanly.

Frequently asked questions

Will a bigger tire make my speedometer read low or high? A larger overall diameter makes the speedometer read low — you’re going faster than it shows — because the tire turns fewer times per mile than the speedo expects.

How much diameter change is too much? Stay within roughly ±3% of the original overall diameter. Past that, speedometer and odometer error climb and fitment and electronics issues become likely.

Does a wider tire always have a bigger diameter? No. Width and diameter are independent. A 245/40R18 can have nearly the same diameter as a 225/45R17 because the lower aspect ratio offsets the extra width and larger rim. That’s exactly the point of plus sizing — and exactly why you should compare the actual numbers.

Drop your current and prospective sizes into the Tire Size Comparison calculator to see the diameter, speedometer error and clearance change instantly, and you’ll know whether a set of wheels fits before you buy.