Plus sizing explained: larger rims and nominal diameter

What changes in a plus size?

A plus-one setup increases rim diameter by one inch; plus-two increases it by two. To keep the same nominal outside diameter, the two sidewalls together must become shorter by the rim-diameter increase.

overall_diameter = rim_diameter + 2 × sidewall_height
sidewall_height_mm = width_mm × aspect_ratio / 100

Because sidewall height depends on both width and aspect ratio, subtracting a fixed number from the aspect ratio does not guarantee the same outside diameter.

A calculated example

Size	Rim	Nominal diameter	Change from base
215/55R17	17 in	668.3 mm	Base
225/45R18	18 in	659.7 mm	−1.29%

The larger rim and shorter sidewall in this example produce a slightly smaller tire overall. These numbers do not establish that either size is appropriate for a particular vehicle.

How the calculator selects candidates

The plus-sizing calculator tries a limited set of width and aspect-ratio combinations for +1 and +2 rim steps, then keeps candidates within ±3% nominal diameter difference. It is a geometric filter; it does not search tire inventory or verify an installation.

Trade-offs to investigate

Change	What to compare in the actual products
Shorter sidewall	Ride comfort, impact protection, and handling characteristics.
Different wheel	Weight, approved tire widths, offset, brake clearance, and load capacity.
Different tire model	Wet, dry, and winter performance; noise; load and speed rating; actual dimensions.
Purchase cost	Tires, wheels, installation, sensors, and any required vehicle setup.

Check the complete installation

Confirm the tire model's approved rim range, the vehicle's load and speed requirements, wheel offset, and clearance through steering and suspension travel. Vehicle electronics and drivetrain requirements also need vehicle-specific verification. A close diameter match cannot answer those questions.

Explore nominal plus-size candidates →