TireFitLab

33x12.50 R18

What size is 33x12.50 R18?

33x12.50 R18 has an overall diameter of about 33.00 in, a section width of 12.50 in and a sidewall height of 7.50 in. It turns roughly 611 times per mile. Below are the exact figures plus plus/minus sizes that stay within the selected diameter tolerance.

Geometric comparison

33x12.50 R15 stays within the configured tolerance for 33x12.50 R18: 0.0% diameter and 0.0% speedometer effect.

Geometric comparison
Metric This size: 33x12.50 R18 Alternative: 33x12.50 R15 Difference Verdict
Overall diameter 33.00 in 33.00 in 0.0% Within tolerance
Section width 12.50 in 12.50 in 0.00 in Computed difference
Sidewall height 7.50 in 9.00 in +1.50 in Computed difference
Circumference 103.67 in 103.67 in 0.0% Within tolerance
Speedo 0.0% 0.0% 0.0% Within tolerance

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What this geometry means

33x12.50 R18 has a mid-range sidewall proportion. The marking directly specifies a 33.00 in nominal overall diameter, a 12.50 in section and a 18-inch rim. One sidewall is 7.50 in; both sidewalls make up 45% of the diameter, while the rim makes up 55%.

At the nominal 103.67 in circumference, 1,000 rotations cover 2.63 km (1.64 mi); the inverse rates are 380/km and 611/mi.

Section-width mix versus 33x12.50 R18: 6 narrower, 2 equal-width and 0 wider entries.

The closest diameter entry is 33x12.50 R15 at +0.0%. Relative to 33x12.50 R18, it changes nominal section width by +0.00 in, each sidewall by +1.50 in and radius by +0.00 in.

Curated alternatives: 8 sizes, spanning -2.8% to +0.5% in nominal diameter versus 33x12.50 R18.

Width endpoints in that set are 275/70 R18 (-1.67 in) and 33x12.50 R16 (+0.00 in) relative to 33x12.50 R18.

This is a nominal geometry comparison only. Vehicle clearance, wheel width and offset, load and speed ratings, and manufacturer approval still require separate checks.

Specification

Specification
Overall diameter 33.00 in
Section width 12.50 in
Sidewall height 7.50 in
Circumference 103.67 in
Revolutions 380 /km · 611 /mi

In-tolerance sizes

Alternative sizes that stay within the selected diameter tolerance:

In-tolerance sizes
Size Diameter Speedo Verdict
33x12.50 R15 +0.0% +0.0% ✓ OK
33x12.50 R16 +0.0% +0.0% ✓ OK
275/70 R18 +0.5% +0.5% ✓ OK
285/75 R16 -0.5% -0.5% ✓ OK
305/45 R22 -0.6% -0.6% ✓ OK
285/65 R18 -1.3% -1.3% ✓ OK
285/45 R22 -2.7% -2.7% ✓ OK
275/65 R18 -2.8% -2.8% ✓ OK

Compare sizes

Looks valid
Looks valid
Units
High fitment riskWithin tolerance · Diameter +0.0% (+0.00 in) · US (~3%)
  • High fitment risk · This rating covers tire geometry only, not vehicle compatibility.
  • Diameter +0.0% · Ride-height change +0.00 in.
  • Speedometer unchanged · 60 mph → 60.0 mph.
  • Sidewall +1.50 in · Width +0.00 in.
Diameter+0.0%+0.00 in
Speedo @ 60+0.0%60 mph → 60.0 mph
Sidewall+1.50 inWidth +0.00 in
Ride-height change+0.00 inDiameter ÷ 2
How this result is calculated

Computed from nominal tire-size geometry: diameter, circumference and speedometer error. It does not check vehicle placard approval, load/speed rating, rim-width range, wheel-arch, suspension or brake clearance.

Methodology

Before fitting

  • Check the vehicle placard and owner’s manual first.
  • Match or exceed the required load index and speed rating.
  • Confirm rim-width range, bolt pattern, hub bore, offset and fasteners.
  • Inspect wheel-arch, suspension and brake clearance under real vehicle load.
  • Check TPMS, cold pressure and local roadworthiness rules.
Diameter
33.00 in 33.00 in+0.00 in
Sidewall
7.50 in 9.00 in+1.50 in
Width
12.50 in 12.50 in+0.00 in
Circumference
103.67 in 103.67 in+0.0%
Revs / mile
611 611
Speedo @ 60
60 mph 60.0 mph+0.0%
Ride-height change
0.00 in +0.00 in
True distance per 100 indicated
100 mi 100.0 mi+0.0%
Δ⌀ +0.00 inΔ↔ +0.00 in
  • Current — 33x12.50 R18
  • New — 33x12.50 R15
  • Grounded to the road: the axle rises +0.00 in.

Result report

33x12.50 R18 33x12.50 R15 · High fitment risk · Within tolerance

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Geometric estimate. Always verify physical clearance (arches, suspension, brakes) before fitting.

In-tolerance alternative sizes

In-tolerance alternative sizes
SizeDiameterSpeedo @ 100Use this
OE 345/55R18-0.2%-0.2%
+1″ 355/50R19-0.1%-0.1%

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Last reviewed: 2026-07-22

Technical check
2026-07-22
Content updated
2026-07-22
What changed
  • Added a localized engine-derived geometry signature and matching Markdown narrative.
Methodology