This difference is within the commonly-cited ±3% tolerance many tire shops use as a rule of thumb for a safe fitment.
The new tire is larger, so your speedometer and odometer will under-report your real speed and distance.
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How This Calculator Works
A tire’s size code (like 225/50R17) encodes its width, sidewall height, and rim size — but not
its overall diameter, which is what actually determines how a tire swap affects your speedometer
and ride height. Enter an original tire size and a replacement size to see exactly how much the
overall diameter changes.
The three numbers in a tire size mean:
225 — section width in millimeters (how wide the tire is).
50 — aspect ratio, the sidewall height as a percentage of the width (not a fixed number).
17 — rim diameter in inches (the wheel it mounts on).
Because the sidewall height is a percentage of width, two tires with the same aspect ratio number
can have very different actual sidewall heights if their widths differ — which is exactly why a
“same-looking” size change can still shift the overall diameter more than expected.
Overall diameter (mm) = Overall Diameter=Rim Diameter+2×Sidewall Height — the
sidewall appears twice, once above the rim and once below.
Circumference = Circumference=Overall Diameter×π, and revolutions per mile =
Circumference63,360 (there are 63,360 inches in a mile).
Since a car’s speedometer and odometer are calibrated to the original tire’s revolutions per
mile, a replacement tire with a different diameter makes the car travel a different real
distance per wheel revolution than the speedometer assumes — throwing off both readings by
roughly the same percentage as the diameter change.
Worked Example
Comparing a 225/50R17 (original) to a 235/45R18 (a common “plus-one” wheel upgrade):
Original (225/50R17)
New (235/45R18)
Sidewall height
112.5 mm
105.75 mm
Overall diameter
25.86 in
26.33 in
The new tire is about 1.8% larger in overall diameter — small enough to fall within the
±3% tolerance many tire shops use as a rule of thumb for a safe fitment, but still enough to make
the speedometer read about 1.8% slower than the car’s true speed.
Cómo funciona esta calculadora
El código de tamaño de una llanta (como 225/50R17) codifica su ancho, la altura del flanco y el
tamaño del rin — pero no su diámetro total, que es lo que realmente determina cómo un cambio de
llanta afecta tu velocímetro y la altura de manejo. Ingresa un tamaño de llanta original y un
tamaño de reemplazo para ver exactamente cuánto cambia el diámetro total.
Los tres números en un tamaño de llanta significan:
225 — el ancho de la sección en milímetros (qué tan ancha es la llanta).
50 — la relación de aspecto, la altura del flanco como porcentaje del ancho (no un número
fijo).
17 — el diámetro del rin en pulgadas (la rueda sobre la que se monta).
Debido a que la altura del flanco es un porcentaje del ancho, dos llantas con el mismo número de
relación de aspecto pueden tener alturas de flanco reales muy diferentes si sus anchos difieren —
razón exacta por la cual un cambio de tamaño que “se ve igual” aún puede alterar el diámetro total
más de lo esperado.
La fórmula
Altura del flanco (mm) = Altura del flanco=Ancho×100Relacioˊn de aspecto.
Diámetro total (mm) = Diaˊmetro total=Diaˊmetro del rin+2×Altura del flanco — el
flanco aparece dos veces, una vez arriba del rin y otra vez abajo.
Circunferencia = Circunferencia=Diaˊmetro total×π, y revoluciones por milla =
Circunferencia63,360 (hay 63,360 pulgadas en una milla).
Dado que el velocímetro y el odómetro de un auto están calibrados según las revoluciones por
milla de la llanta original, una llanta de reemplazo con un diámetro diferente hace que el
auto recorra una distancia real diferente por cada revolución de la rueda de lo que el
velocímetro asume — desviando ambas lecturas en aproximadamente el mismo porcentaje que el
cambio de diámetro.
Ejemplo resuelto
Comparando una 225/50R17 (original) con una 235/45R18 (una mejora común de rueda tipo
“plus-one”):
Original (225/50R17)
Nueva (235/45R18)
Altura del flanco
112.5 mm
105.75 mm
Diámetro total
25.86 in
26.33 in
La llanta nueva es aproximadamente 1.8% más grande en diámetro total — lo suficientemente
pequeño para estar dentro de la tolerancia de ±3% que muchos talleres de llantas usan como regla
general para un ajuste seguro, pero suficiente para hacer que el velocímetro marque
aproximadamente 1.8% más lento que la velocidad real del auto.
What do the numbers in a tire size like 225/50R17 mean?
225 is the section width in millimeters, 50 is the aspect ratio (the sidewall height as a percentage of the width), R stands for radial construction (how the layers of fabric inside the tire are arranged — the standard construction for virtually every tire sold today, as opposed to older diagonal-bias construction), and 17 is the rim diameter in inches that the tire mounts on.
How much tire size difference is considered safe?
Many tire shops use a commonly-cited rule of thumb of keeping the overall diameter within about ±3% of the original size — small enough to avoid meaningfully throwing off the speedometer, odometer, or ABS/traction control systems, which all rely on the wheel's expected rotation speed.
Why does a different tire size affect my speedometer?
A car's speedometer counts wheel rotations and converts that to a speed using the original tire's circumference. A different-diameter tire covers a different real distance per rotation, so the same rotation speed now corresponds to a different actual speed than what's displayed.
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