CompareWheels
Calculation methods & limits
CompareWheels computes nominal geometry from the numbers you enter. Results are estimates of dimensional changes; we do not certify fitment or recommend a tire for a vehicle.
Tire dimensions
For W/A R D: section width W is millimetres, aspect ratio A is a percentage, and rim diameter D is inches. Sidewall height H = W × A / 100. Overall diameter = 2H + 25.4D. Circumference = π × diameter. Revolutions per mile = 1,609,344 / circumference in mm; revolutions per kilometre = 1,000,000 / circumference.
Inch notation such as 33x12.50R15 supplies nominal overall diameter and width directly. A calculated aspect ratio is only a geometric description, not proof that a matching metric tire exists.
Speed and radius
Diameter change (%) = 100 × (new diameter / current diameter − 1). Estimated speed = indicated speed × new diameter / current diameter. This assumes the current tire is the calibration reference and excludes existing speedometer bias. Radius change = (new diameter − current diameter) / 2; this is not a prediction of loaded ride height.
Wheel width, offset and spacers
Width in mm = nominal wheel width in inches × 25.4. Outward bead-seat position = width / 2 − offset. Inward position = width / 2 + offset. The difference between proposed and current positions gives edge movement. Positive outward movement means more protrusion; positive inward movement means reduced suspension-side clearance.
A spacer reduces effective offset: effective ET = wheel ET − spacer thickness. Nominal backspacing = nominal width / 2 + ET (use consistent units). Physical backspacing is measured to the outer rim flange and requires the actual overall width. Our converter excludes that flange; do not treat its result as a measured physical backspacing.
Axle track change = 2 × (current ET − proposed effective ET), assuming identical changes on both sides of one axle. Changing width without changing offset does not move the wheel centreline. If your current setup has spacers, enter its effective offset.
3D visualization
The original Blender asset is reshaped to the nominal overall tire diameter, section width and rim diameter. Wheel width scales the illustrative rim barrel. The two wheels share an orthographic camera, so perspective does not make one appear larger. Their 3D presentation does not depict offset movement; use the mounting-face-aligned Position view for that comparison. In tire-only mode the rim width is an illustrative value derived from tire width, not a permitted rim-width recommendation.
Tread, spokes, hub details and sidewall shape are generic styling. The model is not a particular product, manufacturer specification, approved assembly or simulation of tire stretch.
What these diagrams do not model
The side view uses nominal diameters at the same scale. The Position view uses millimetre coordinates and a uniform scale in both axes, with the vehicle mounting face at zero and positive coordinates inboard. Wheel centre = wheel ET − spacer thickness; outboard bead seat = centre − half the wheel width; inboard bead seat = centre + half the wheel width. A proposed spacer moves its wheel mounting face outward by its thickness. The displayed rim lips are illustrative and excluded from edge-change measurements. Tire-section outlines use nominal section widths centered on the wheel centerline; they are not manufacturer measurements. Tire shoulders, casing thickness, rim profiles, wheel discs, hub and the faded suspension reference are illustrative. When entered wheel width exceeds nominal tire section width, a dashed envelope replaces the casing; this is not a prediction of the mounted tire shape. The suspension is not positioned from measured vehicle data and must not be used to judge clearance. It does not model tire stretch, actual tire shape, rim flange thickness, brake clearance, steering travel, suspension compression, body clearance or scrub radius. Tire dimensions vary with product, rim width, load, pressure and wear.
Reproduce a result
For 205/55R16, sidewall height is 205 × 55 / 100 = 112.75 mm, and diameter is 2 × 112.75 + 16 × 25.4 = 631.9 mm. An 8-inch ET35 wheel has bead-seat width 203.2 mm, outward position 101.6 − 35 = 66.6 mm, and inward position 101.6 + 35 = 136.6 mm. Adding a 10 mm spacer changes effective ET to 25: outward position becomes 76.6 mm and inward position 126.6 mm.
Try these inputs in the tire calculator, offset calculator and spacer calculator. Each describes nominal geometry, not approval of an installation.
Verification
Automated reference tests cover tire geometry, speed ratios, spacer effects, centreline track changes and invalid inputs. These are software checks, not an independent automotive certification. Confirm actual specifications with the tire and vehicle manufacturers and have physical clearance checked before installation.
Reference material
Tire Rack: calculating tire dimensions explains nominal size calculations and why actual dimensions differ.
Bridgestone passenger and light truck tire resource manual provides wheel terminology and dimensional reference. Manufacturer guidance takes precedence over this tool.