Full-Size Spare Matching Tool
Check that a full spare matches the other four. Use the calculator below. Results update as you type.
How the Full-Size Spare Matching Tool Works
This tool works by checking a spare wheel and tire against 12 specifications of the road wheels. A full-size spare must match closely enough to run without speed or distance limits.
- Enter the road tire size, load index and speed rating.
- Enter the road wheel's width, offset, bolt pattern and center bore.
- Enter the same specifications for the spare.
- Read each specification as Match or Mismatch.
The Formula
The formula compares each spare value with the road value and applies a pass rule per specification.
Diameter Gap (%) = (D_spare − D_road) ÷ D_road × 100 Load Index: spare ≥ road Speed Rating: spare ≥ road Offset Gap (mm) = |ET_spare − ET_road|Worked example: a road wheel at ET44 and a spare at ET40 differ by 4 mm, which passes a 5 mm rule. A spare with load index 112 against road tires at 115 fails.
Conversion Table
The conversion table lists 12 specifications a full-size spare must match. Each row gives an example value and the allowed variation.
The 4 wheel rows (bolt pattern, center bore, lug seat and offset) decide whether the spare mounts safely. The 4 tire rows (size, load, speed and load type) decide whether it carries the vehicle safely.
| Specification | Example Value | Allowed Variation |
|---|---|---|
| Tire size | 265/70R17 | Same size |
| Overall diameter | 31.6 in / 803 mm | Match tread depth within AWD limit |
| Load index | 115 | Equal or higher |
| Speed rating | T (190 km/h, 118 mph) | Equal or higher |
| Load type | SL / XL / LT | Same |
| Rim diameter | 17 in | Same |
| Rim width | 7.5 in | Within approved range |
| Offset | +44 mm | Same or within 5 mm |
| Bolt pattern | 6x139.7 (6x5.5) | Same |
| Center bore | 106.1 mm | Same or hub-centric ring |
| Lug seat | 60° conical | Same |
| TPMS sensor | 315 MHz | Same frequency or none |
What Is the Full-Size Spare Matching Tool?
The Full-Size Spare Matching Tool is a checklist tool that confirms a full-size spare matches the vehicle's road wheels and tires. A matching full-size spare lets you join it into a 5-tire rotation.
Many trucks and SUVs carry a full-size spare on a steel wheel with a different offset or width than the alloy road wheels. The tool shows which of those differences matter.
Common Uses
This tool has 4 uses: 5-tire rotations, spare purchases, overland builds and wheel swaps.
- Run 5-tire rotations: Confirm the spare can join the rotation.
- Buy a spare: Pick a matching wheel and tire.
- Build overland rigs: Match a spare to upsized road tires.
- Swap wheels: Check an aftermarket wheel against the spare carrier.