A Free Tool · Compare Two Sizes · Speedometer Error
What does a new tire size actually change?
Enter two tire sizes — like 225/45R17 and 245/40R18 —
and get a side-by-side comparison of overall diameter (in inches and millimeters), sidewall
height, circumference, and revolutions per mile, plus the percent difference and how far off
your speedometer will read at 30, 60, and 70 mph. The numbers are pure geometry from the
P-metric size on the sidewall.
Read this first
These are geometric estimates from the printed tire size, not a fitment or safety guarantee.
The real mounted diameter varies a little by brand and model, tread depth, load, and inflation
pressure, so treat the result as a close comparison rather than an exact measurement. Staying
within about ±3% overall diameter is the common rule of thumb for a replacement set;
larger changes can throw off the speedometer, odometer, and electronics, and raise clearance
questions this tool does not check. Confirm fitment against your vehicle's specifications.
Enter your current size and the size you're considering, in the form printed on the sidewall (width/aspect R wheel). You'll get a full comparison and your speedometer error, recalculated as you type.
Format: width(mm)/aspect(%)R wheel(in) — e.g. 225/45R17 or 225/45/17.
The replacement you're comparing against tire 1.
Measurement
Tire 1225/45R17
Tire 2245/40R18
Overall diameter (in)
Overall diameter (mm)
Sidewall height (mm)
Circumference (in)
Revolutions per mile
Speedometer reads vs. actual speed
Speedometer shows
You're actually going
Off by
The math, honestly
How the comparison is figured
It's all geometry from the size code. A P-metric size like 225/45R17 gives the
section width in millimeters (225), the aspect ratio as a
percent (45), and the wheel diameter in inches (17). The sidewall height is
width × (aspect / 100) — here 225 × 0.45 = 101.25 mm.
The overall diameter is the wheel plus two sidewalls (top and bottom):
wheel_in × 25.4 + 2 × sidewall_mm. For 225/45R17 that's
17 × 25.4 + 2 × 101.25 = 634.30 mm, which divided by 25.4 is
24.97 in. Circumference is π × diameter_in, and
revolutions per mile is 63360 / circumference_in (there are
63,360 inches in a mile).
The percent difference is
(dia₂ − dia₁) / dia₁ × 100. And because your speedometer
is calibrated to tire 1, the actual speed at an indicated reading S is
S × (dia₂ / dia₁) — a taller new tire means
you're really going faster than the dash shows. Going 225/45R17 → 245/40R18 is
+3.0% on diameter, so an indicated 60 mph is actually about 61.8 mph.
Reading a P-metric tire size
Every part of a size like 225/45R17 is one measurement. Here's what each piece
means and what unit it's in — the same parts this calculator parses out of the string.
Part
In 225/45R17example value
What it means
Width
225
Section width in millimeters — widest point of the tire.
Aspect
45
Sidewall height as a percent of the width (here 45% of 225 mm).
R
R
Radial construction — the standard for passenger tires.
Wheel
17
Wheel (rim) diameter in inches the tire mounts on.
The width and aspect ratio describe the rubber; the wheel number describes the rim. Two tires
can share a wheel size and still differ a lot in overall height when the width or aspect ratio
changes. The calculator accepts both 225/45R17 and 225/45/17.
Common plus-size comparisons
A few common “plus-size” swaps — a bigger wheel with a shorter sidewall —
computed with the same geometry as the tool. Notice how the good swaps land within about
±3% on overall diameter, which keeps the speedometer close to accurate.
From → To
Diameterfrom (in)
Diameterto (in)
Differenceoverall diameter
225/45R17 → 245/40R18
24.97
25.72
+3.0%
205/55R16 → 225/45R17
24.88
24.97
+0.4%
215/60R16 → 225/50R17
26.16
25.86
−1.1%
235/75R15 → 265/70R16
28.88
30.61
+6.0%
The first three swaps stay inside the ±3% rule of thumb. The last one — a bigger
light-truck tire — is +6.0%, which would read the speedometer low and likely needs a
clearance and fitment check. Run any pair through the calculator above to see the speedometer
error.
Reading the result well
A diameter difference is only useful if you know what it does to the car. Four things worth
knowing before you buy.
Stay within about ±3% on diameter
For a straight replacement set, keeping the new overall diameter within roughly 3% of the original is the common rule of thumb. That keeps the speedometer, odometer, and electronic systems — ABS, traction control, and on many cars the transmission shift points — close to what they expect. Bigger jumps are sometimes done deliberately on trucks and off-road builds, but they come with recalibration and clearance work.
A taller tire makes the speedometer read low
The speedometer counts wheel rotations and assumes the factory tire's diameter. A taller tire covers more ground per turn, so your true speed is higher than the dash shows; a shorter tire reads the opposite. The error scales directly with the diameter ratio — a 3% larger tire means you're going about 3% faster than indicated, and your odometer logs about 3% fewer miles than you actually drive.
Plus sizing trades sidewall for wheel
Plus sizing means going up a wheel size while dropping the aspect ratio so the overall diameter stays close. The shorter sidewall sharpens steering response and lets you run a wider tread, at the cost of a firmer ride and more vulnerability to potholes. The goal is to keep the diameter within a few percent so nothing else on the car has to change.
Width and clearance are a separate question
This calculator compares diameter, circumference, and revolutions per mile — the things that drive your speedometer. It does not check whether a wider tire physically fits: that depends on wheel width, offset, and clearance in the wheel well and at full steering lock. Before buying a wider or much taller tire, confirm fitment against your vehicle's specifications or a tire-and-wheel fitment guide.
Where to buy
Got your numbers? Here's where to pick up what you need:
The terms behind the calculator, in plain English. These are background definitions, not
fitment specifications — confirm any change against your vehicle's documentation.
Section width
The first number in the size, in millimeters — the distance across the widest part of the tire (sidewall to sidewall). In 225/45R17 the section width is 225 mm. It's not the same as the tread width, which is narrower.
Aspect ratio
The second number, a percentage. The sidewall height equals the aspect ratio times the section width. A 45 means the sidewall is 45% of 225 mm, about 101 mm. Lower numbers mean a shorter, sportier sidewall; higher numbers mean a taller, cushier one.
Sidewall height
The vertical rubber between the wheel and the tread, computed as width × (aspect / 100) in millimeters. It counts twice in the overall diameter — once at the top and once at the bottom of the wheel.
Wheel diameter
The last number in the size, in inches — the diameter of the rim the tire mounts on. A 17 means a 17-inch wheel. It's the only part of the P-metric size given in inches rather than millimeters.
Overall diameter
The full height of the mounted tire: wheel_in × 25.4 + 2 × sidewall_mm, then divided by 25.4 to read it in inches. This is the number that matters for the speedometer, because it sets how far the tire rolls per turn.
Circumference
The distance around the tire, π × overall diameter. It's the rolling distance per revolution, which links the tire's size to revolutions per mile and ultimately to the speedometer reading.
Revolutions per mile
How many times the tire turns to cover a mile: 63360 / circumference_in (a mile is 63,360 inches). A taller tire has a larger circumference and so fewer revolutions per mile, which is why it makes the speedometer and odometer read low.
Plus sizing
Moving to a larger wheel while lowering the aspect ratio so the overall diameter stays close to the original. “Plus one” is up one inch of wheel. Done right, it keeps the speedometer accurate while sharpening handling.
Frequently asked
It's three measurements in one code. 225 is the section width in millimeters, the distance across the widest part of the tire. 45 is the aspect ratio: the sidewall height is 45% of the width, so 225 × 0.45 = 101.25 mm. The R means radial construction, and 17 is the wheel diameter in inches. Put together, the overall diameter is 17 × 25.4 + two sidewalls, about 634 mm or 24.97 in. Try it in the calculator.
225 is the tread width in millimeters. 45 is the aspect ratio, meaning the sidewall is 45% of 225 mm, or about 101 mm tall. R stands for radial. 17 is the wheel diameter in inches. The first two numbers describe the rubber and the last describes the wheel it mounts on — so two tires with the same 17 can have very different overall heights depending on the width and aspect ratio.
Yes. Your speedometer counts wheel rotations and assumes the original tire's diameter. A taller tire travels farther per rotation, so you're actually going faster than the dash shows; a shorter tire reads fast. The error is the diameter ratio: actual speed = indicated speed × (new diameter / old diameter). Going from a 24.97-inch tire to a 25.72-inch tire, an indicated 60 mph is really about 61.8 mph, roughly 3% fast. Your odometer drifts by the same percentage.
For a straight replacement, keeping the overall diameter within about ±3% of the original is the common rule of thumb. That keeps the speedometer, odometer, and electronics close to accurate, and usually avoids rubbing. Fitting a much larger tire is a separate question about width, wheel offset, and clearance in the wheel well — which this calculator does not check. Always confirm fitment against your vehicle's specifications before buying.
Plus sizing is moving to a larger-diameter wheel while keeping the overall tire diameter roughly the same. You go up in wheel size and down in aspect ratio at the same time, so the shorter sidewall offsets the bigger rim. Going from 225/45R17 to 245/40R18, for example, raises the wheel an inch and trims the sidewall, landing within about 3% on overall diameter. The point is sharper handling and a wider tread without throwing off the speedometer.
Revolutions per mile is how many times a tire turns to cover one mile: 63,360 inches in a mile divided by the tire's circumference in inches. A 24.97-inch tire turns about 808 times per mile; a taller 25.72-inch tire turns about 784 times. Fewer revolutions per mile means the wheel-speed sensors count slower for the same road speed — which is exactly why a taller tire makes the speedometer and odometer read low.
This calculator computes the geometric diameter straight from the P-metric size, which is what the markings describe. The actual mounted diameter varies a little by brand and model, tread depth, the load on the tire, and inflation pressure. Manufacturers publish a measured overall diameter and revs-per-mile spec that can sit a fraction of an inch off the geometric figure. Treat this result as a close, consistent estimate for comparing sizes — not an exact mounted measurement.
Common mistakes
Tire size errors are mostly misread number positions on the sidewall, or missed consequences of changing to a different overall diameter.
Reading the aspect ratio as a height in inches
In a size like 225/55R17, the 55 is the aspect ratio — the sidewall height as a percentage of the tread width, not a measurement in inches or millimeters. Sidewall height in mm = 225 × 0.55 = 123.75 mm. Treating 55 as a direct inch or mm measurement produces a completely wrong tire height and diameter calculation.
Assuming a different tire size has no effect on the speedometer
Speedometer and odometer readings are calibrated to a specific tire's rolling circumference. Switching to a tire with a larger overall diameter means the wheel travels farther per revolution than the vehicle's computer expects — your actual speed will be higher than displayed, and odometer mileage will read low. The speedometer error percentage = (new diameter − stock diameter) ÷ stock diameter × 100. Even a 1-inch difference in overall diameter introduces roughly a 2–3% speedometer error.
Choosing a "plus-size" fitment without checking clearance
Upsizing the wheel diameter (going from 17" to 18" wheels, for example) requires a proportionally lower aspect ratio to keep the overall tire diameter close to stock. If the overall diameter increases significantly, the tire may contact the wheel well, suspension components, or brake hardware under full lock or over bumps. A tire size calculator shows the diameter change — verify the result falls within the vehicle manufacturer's recommended fitment range before purchasing.
Comparing tires by rim size alone, ignoring overall diameter
Two tires with the same rim diameter can have very different overall diameters if their aspect ratios differ. A 225/65R17 and a 245/40R17 both mount on a 17" rim, but their overall diameters differ by nearly 3 inches. Load rating, rolling circumference, and fit depend on the full size specification, not just the wheel diameter.