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235/40R18 tires: size explained

235/40R18

235/40R18 is a Euro-metric tire size. It is about 25.4 in (645 mm) tall, 235 mm wide, and made for a 18-inch wheel.

What 235/40R18 means

A tire size describes the tire's shape and the wheel it fits. It says nothing about load capacity or speed capability, which are printed separately on the sidewall.

No letterEuro-metric
With no letter in front of the width, this is a Euro-metric size (also called hard metric). It has the same dimensions as the P-metric version but is built to a different load standard, so its load capacity can differ.
235Section width
The tire's width is 235 millimeters (9.3 in), measured from sidewall to sidewall.
40Aspect ratio
The sidewall height is 40% of the width. For 235 mm that is about 3.7 in (94 mm). A smaller number means a shorter sidewall.
RRadial construction
R means radial construction, where the internal cords run across the tire from one edge to the other.
18Wheel (rim) diameter
The tire is made for a wheel 18 inches across. Its overall diameter is larger: about 25.4 in (645 mm).

235/40R18 dimensions

25.4 inOverall diameter: 25.4 in (645 mm)Rim diameter: 18.0 in (457 mm)Sidewall height: 3.7 in (94 mm)9.3 inSection width: 9.3 in (235 mm)Sidewall height: 3.7 in (94 mm)
Each drawing is to scale on its own. The curves of the tread shoulder and bead are simplified; the table below has the same numbers.
Section width
9.3 in (235 mm)
Sidewall height
3.7 in (94 mm)
Wheel (rim) diameter
18.0 in (457 mm)
Overall diameter
25.4 in (645 mm)
Circumference
79.8 in (2027 mm)
Revolutions per mile
794

These are nominal design dimensions. Real tires vary by brand, tread depth, wheel width, load and pressure.

Which vehicles use 235/40R18?

These vehicles list 235/40R18 among their verified sizes, by trim. Open one to see all of its sizes.

Shop for 235/40R18 tires

Confirm the size on your tire sidewall or door placard first. These links open each store's own search for 235/40R18; prices and stock are on their sites.

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Snow chains and tire socks for 235/40R18

Where winter roads call for extra traction, chains and tire socks are two options that fit over your tires. They are made for specific tire sizes, so these links open Amazon searches for 235/40R18. Whether they suit your vehicle depends on its clearance and the local rules.

Snow chains for 235/40R18 (Amazon search, opens in a new tab)Tire socks for 235/40R18 (Amazon search, opens in a new tab)

Paid links to Amazon search results. As an Amazon Associate I earn from qualifying purchases. Read our affiliate disclosure.

Before you buy

  • Chains and socks are sold by tire size, so match the size on your sidewall exactly.
  • Check your owner's manual first: many newer cars and SUVs limit or prohibit chains, or require low-profile (often called SAE Class S) chains because of tight wheel-well clearance.
  • Tire socks are a lighter-duty option; check your state's rules, because some areas only accept chains when chain controls are in effect.
  • Put them on the drive wheels (or as your manual says) and follow the maker's speed limit.

Similar sizes

These common sizes have an overall diameter within about 3% of 235/40R18. A common rule of thumb is to keep a replacement size within about 3% of the original overall diameter. Confirm any size change with your vehicle's owner's manual or a tire professional.

Common sizes with an overall diameter close to 235/40R18
SizeWheel diameterDiameter differenceCalculator
205/60R1616 in (different)+1.12%Compare 235/40R18 with 205/60R16
195/65R1515 in (different)-1.66%Compare 235/40R18 with 195/65R15
225/45R1717 in (different)-1.69%Compare 235/40R18 with 225/45R17
225/50R1717 in (different)+1.80%Compare 235/40R18 with 225/50R17
205/55R1616 in (different)-2.06%Compare 235/40R18 with 205/55R16
215/60R1616 in (different)+2.98%Compare 235/40R18 with 215/60R16

Same dimensions, different load standard

P235/40R18 (P-metric) has the same dimensions as 235/40R18, but it is built to a different load standard. Check the load index on the sidewall rather than assuming the two are interchangeable.

Compare 235/40R18 with similar sizes

Side-by-side dimensions and speedometer difference for sizes close to 235/40R18 in overall diameter.

Compare 235/40R18 with another size

See how another size differs in height, width, sidewall and speedometer reading in the tire size calculator.

Compare 235/40R18 with another size

Questions about 235/40R18

What does 235/40R18 mean?

235/40R18 describes the shape and size of a tire, not how much it can carry or how fast it can go. 235 is the width in millimeters, 40 is the sidewall height as a percentage of that width (about 94 mm), R is the construction type and 18 is the wheel diameter in inches.

How tall is a 235/40R18 tire?

A 235/40R18 tire has a nominal overall diameter of about 25.4 in (645 mm), with a sidewall about 3.7 in (94 mm) tall. These are nominal design dimensions. Real tires vary by brand, tread depth, wheel width, load and pressure.

How many revolutions per mile does a 235/40R18 tire make?

About 794 revolutions per mile, based on a circumference of 79.8 in (2027 mm). A vehicle's speedometer and odometer are calibrated to the tire size it was built with, so a size with a different overall diameter changes what they read. A common rule of thumb is to keep a replacement size within about 3% of the original overall diameter. Confirm any size change with your vehicle's owner's manual or a tire professional.

Is 235/40R18 the same as P235/40R18?

They have the same dimensions: both are about 25.4 in (645 mm) tall and 235 mm wide. They are built to different load standards, though (Euro-metric and P-metric), so the load a tire can carry can differ. Check the load index on the sidewall, and ask a tire professional or your owner's manual before treating them as interchangeable.

Which sizes have a similar diameter to 235/40R18?

Among the common sizes we list, these are within about 3% of the overall diameter of 235/40R18: 205/60R16 (+1.12%), 195/65R15 (-1.66%), 225/45R17 (-1.69%), and 3 more. Diameter is only one measure. Width, wheel diameter, load capacity and clearance matter too. Confirm any size change with your vehicle's owner's manual or a tire professional.

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Paid links to Amazon search results. As an Amazon Associate I earn from qualifying purchases. Read our affiliate disclosure.