315/40R18 tires: size explained

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.

315Section width

The tire's width is 315 millimeters (12.4 in), measured from sidewall to sidewall.

40Aspect ratio

The sidewall height is 40% of the width. For 315 mm that is about 5.0 in (126 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 27.9 in (709 mm).

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

These vehicles list 315/40R18 among their verified sizes, by trim. Open one to see all of its sizes.

• 2022 Dodge Challenger SRT Super Stock
• 2021 Dodge Challenger SRT Super Stock
• 2020 Dodge Challenger SRT Super Stock

Confirm the size on your tire sidewall or door placard first. These links open each store's own search for 315/40R18; prices and stock are on their sites.

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 315/40R18. Whether they suit your vehicle depends on its clearance and the local rules.

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.

These common sizes have an overall diameter within about 3% of 315/40R18. 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.

Side-by-side dimensions and speedometer difference for sizes close to 315/40R18 in overall diameter.

• 285/35R20 vs 315/40R18-0.24% shorter than 315/40R18
• 215/70R16 vs 315/40R18-0.25% shorter than 315/40R18
• 215/65R17 vs 315/40R18+0.30% taller than 315/40R18
• 255/35R21 vs 315/40R18+0.38% taller than 315/40R18
• 255/40R20 vs 315/40R18+0.39% taller than 315/40R18
• 255/45R19 vs 315/40R18+0.41% taller than 315/40R18
• 315/40R18 vs 345/25R21-0.47% shorter than 315/40R18
• 235/60R17 vs 315/40R18+0.65% taller than 315/40R18
• 245/45R19 vs 315/40R18-0.86% shorter than 315/40R18
• 275/40R19 vs 315/40R18-0.93% shorter than 315/40R18

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