33×12.5R15 vs 31×10.5R15
31×10.5R15 has an overall diameter of 787.4 mm (31.00") compared with 838.2 mm (33.00") for 33×12.5R15, a difference of -50.8 mm (-6.06 %). The sidewall changes by -25.4 mm, section width by -51 mm and ride height by -25.4 mm. Using the ETRTO rolling circumference, 33×12.5R15 turns 391 times per kilometre and 31×10.5R15 turns 416 times. With 31×10.5R15 fitted to a car calibrated for 33×12.5R15, an indicated 100 km/h is a true 93.9 km/h, and the odometer records 106.5 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 | 33×12.5R15 | 31×10.5R15 | Difference |
|---|---|---|---|
| Section width | 318 mm | 267 mm | -51 mm |
| Sidewall height | 228.6 mm | 203.2 mm | -25.4 mm |
| Rim diameter | 15" | 15" | 0.0" |
| Overall diameter | 838.2 mm (33.00") | 787.4 mm (31.00") | -50.8 mm (-6.06 %) |
| Static circumference | 2633 mm | 2474 mm | -160 mm |
| Rolling circumference (ETRTO) | 2557 mm | 2402 mm | -155 mm |
| Revs per km | 391 | 416 | +25 |
| Ride height | -25.4 mm | ||
| Inch equivalent | 33×12.5R15 | 31×10.5R15 | |
| Approved rim widths (approx.) | 9″–10.5″ | 7″–8.5″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 37.6 | 47.0 | 56.4 | 75.2 | 93.9 | 103.3 |
| Error (km/h) | -2.4 | -3.0 | -3.6 | -4.8 | -6.1 | -6.7 |
Assumes the speedo was accurate on 33×12.5R15. Odometer records 106.5 km for every 100 km actually driven.
- The 31×10.5R15 is 50.8 mm (6.06 %) smaller in diameter, so your speedo will read high and your odometer will over-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 31×10.5R15 legal in Australia? Needs certification
| Overall diameter (VSB 14 LS 4.2.4) 31×10.5R15 is 50.8 mm smaller than 33×12.5R15; the limit without certification is -26 mm. | Cert. |
| Speedometer (ADR 18/03) A smaller tyre makes the speedo read high: at an indicated 100 km/h you would be doing 93.9 km/h (over-read 6.1 km/h; ADR 18 allows up to 13.4 km/h at that speed). | OK |
| 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 31×10.5R15 instead of 33×12.5R15?
Overall diameter changes by -50.8 mm (-6.06 %). 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 31×10.5R15?
Your speedo will read high by about 6.06 %. At an indicated 100 km/h you would actually be travelling 93.9 km/h, and at 110 km/h about 103.3 km/h.
What is the rolling circumference of 31×10.5R15?
Static circumference is 2474 mm; the ETRTO rolling circumference under load is about 2402 mm, which is 416 revolutions per kilometre.
31×10.5R15 in inches
31×10.5R15 is approximately 31×10.5R15: 31.0" tall, 10.5" wide, on a 15" rim, with a 8.0" 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.