31×10.5R15 vs 33×12.5R15
33×12.5R15 has an overall diameter of 838.2 mm (33.00") compared with 787.4 mm (31.00") for 31×10.5R15, a difference of +50.8 mm (+6.45 %). The sidewall changes by +25.4 mm, section width by +51 mm and ride height by +25.4 mm. Using the ETRTO rolling circumference, 31×10.5R15 turns 416 times per kilometre and 33×12.5R15 turns 391 times. With 33×12.5R15 fitted to a car calibrated for 31×10.5R15, an indicated 100 km/h is a true 106.5 km/h, and the odometer records 93.9 km for every 100 km driven. This exceeds the commonly quoted 3 % tolerance; check clearances, ABS/ESC calibration and local roadworthy rules (VSB 14 in Australia).
Full comparison
| Measurement | 31×10.5R15 | 33×12.5R15 | Difference |
|---|---|---|---|
| Section width | 267 mm | 318 mm | +51 mm |
| Sidewall height | 203.2 mm | 228.6 mm | +25.4 mm |
| Rim diameter | 15" | 15" | 0.0" |
| Overall diameter | 787.4 mm (31.00") | 838.2 mm (33.00") | +50.8 mm (+6.45 %) |
| Static circumference | 2474 mm | 2633 mm | +160 mm |
| Rolling circumference (ETRTO) | 2402 mm | 2557 mm | +155 mm |
| Revs per km | 416 | 391 | -25 |
| Ride height | +25.4 mm | ||
| Inch equivalent | 31×10.5R15 | 33×12.5R15 | |
| Approved rim widths (approx.) | 7″–8.5″ | 9″–10.5″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 42.6 | 53.2 | 63.9 | 85.2 | 106.5 | 117.1 |
| Error (km/h) | +2.6 | +3.2 | +3.9 | +5.2 | +6.5 | +7.1 |
Assumes the speedo was accurate on 31×10.5R15. Odometer records 93.9 km for every 100 km actually driven.
- The 33×12.5R15 is 50.8 mm (6.45 %) larger in diameter, so your speedo will read low and your odometer will under-record distance.
- Section width changes by +51 mm. Check guard, strut and inner-arch clearance and that the rim width is within the approved range for the new tyre.
- A diameter change over 3 % is outside the usual rule of thumb; check local roadworthy rules (in Australia see VSB 14 / NCOP Section LS) and ABS/ESC calibration.
Is 33×12.5R15 legal in Australia? Needs certification
| Overall diameter (VSB 14 LS 4.2.4) 33×12.5R15 is 50.8 mm larger than 31×10.5R15; the limit without certification for a passenger car (MA/MB) is +15 mm. Larger increases need an approved signatory / engineering certificate (modification code LS or state equivalent) and may not be permitted at all. | Cert. |
| Speedometer (ADR 18/03) A larger tyre makes the speedo read low: at an indicated 100 km/h you would be doing 106.5 km/h. ADR 18 requires the speedo never to read below true speed; a 6.5 % increase is unlikely to be absorbed by the factory over-read, so the speedo will need recalibration (correction module) and certification. | Cert. |
| Load index (tyre placard / ADR 42) Enter the placard load index and the replacement tyre load index to check. | ? |
| Speed rating (tyre placard / ADR 42) Enter the placard speed rating and the replacement tyre speed rating to check. | ? |
Frequently asked
Can I fit 33×12.5R15 instead of 31×10.5R15?
Overall diameter changes by +50.8 mm (+6.45 %). That is outside the 3 % rule of thumb, so it may rub, upset ABS/ESC calibration and may not be legal without certification. In Australia VSB 14 allows up to +15 mm for passenger cars and +50 mm for 4WDs/goods vehicles without certification.
How much will my speedo be out with 33×12.5R15?
Your speedo will read low by about 6.45 %. At an indicated 100 km/h you would actually be travelling 106.5 km/h, and at 110 km/h about 117.1 km/h.
What is the rolling circumference of 33×12.5R15?
Static circumference is 2633 mm; the ETRTO rolling circumference under load is about 2557 mm, which is 391 revolutions per kilometre.
33×12.5R15 in inches
33×12.5R15 is approximately 33×12.5R15: 33.0" tall, 12.5" wide, on a 15" rim, with a 9.0" sidewall.
Buying tyres for your business?
Onyx Tyres supplies workshops, retailers and fleets across Australia at wholesale prices. Open a trade account to see stock and pricing and order online.