255/50R19 vs 275/40R21
275/40R21 has an overall diameter of 753.4 mm (29.66") compared with 737.6 mm (29.04") for 255/50R19, a difference of +15.8 mm (+2.14 %). The sidewall changes by -17.5 mm, section width by +20 mm and ride height by +7.9 mm. Using the ETRTO rolling circumference, 255/50R19 turns 445 times per kilometre and 275/40R21 turns 435 times. With 275/40R21 fitted to a car calibrated for 255/50R19, an indicated 100 km/h is a true 102.1 km/h, and the odometer records 97.9 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 255/50R19 | 275/40R21 | Difference |
|---|---|---|---|
| Section width | 255 mm | 275 mm | +20 mm |
| Sidewall height | 127.5 mm | 110.0 mm | -17.5 mm |
| Rim diameter | 19" | 21" | +2.0" |
| Overall diameter | 737.6 mm (29.04") | 753.4 mm (29.66") | +15.8 mm (+2.14 %) |
| Static circumference | 2317 mm | 2367 mm | +50 mm |
| Rolling circumference (ETRTO) | 2250 mm | 2298 mm | +48 mm |
| Revs per km | 445 | 435 | -9 |
| Ride height | +7.9 mm | ||
| Inch equivalent | 29x10R19 | 29.7×10.8R21 | |
| Approved rim widths (approx.) | 7″–9″ | 9″–10.5″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 40.9 | 51.1 | 61.3 | 81.7 | 102.1 | 112.4 |
| Error (km/h) | +0.9 | +1.1 | +1.3 | +1.7 | +2.1 | +2.4 |
Assumes the speedo was accurate on 255/50R19. Odometer records 97.9 km for every 100 km actually driven.
- The 275/40R21 is 15.8 mm (2.14 %) larger in diameter, so your speedo will read low and your odometer will under-record distance.
- Section width changes by +20 mm. Check guard, strut and inner-arch clearance and that the rim width is within the approved range for the new tyre.
- Rim diameter changes from 19" to 21", so new wheels are required.
Is 275/40R21 legal in Australia? Needs certification
| Overall diameter (VSB 14 LS 4.2.4) 275/40R21 is 15.8 mm larger than 255/50R19; 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 lower: at an indicated 100 km/h you would be doing 102.1 km/h. ADR 18 requires the speedo never to read below true speed, but factory speedos typically over-read by 2-6 %, so a 2.1 % increase is usually absorbed. Verify with a GPS speed app after fitting. | Verify |
| 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 275/40R21 instead of 255/50R19?
Overall diameter changes by +15.8 mm (+2.14 %). That is close to the 3 % rule of thumb; it may fit, but check guard, strut and brake clearance and your local rules. 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 275/40R21?
Your speedo will read low by about 2.14 %. At an indicated 100 km/h you would actually be travelling 102.1 km/h, and at 110 km/h about 112.4 km/h.
What is the rolling circumference of 275/40R21?
Static circumference is 2367 mm; the ETRTO rolling circumference under load is about 2298 mm, which is 435 revolutions per kilometre.
275/40R21 in inches
275/40R21 is approximately 29.7×10.8R21: 29.7" tall, 10.8" wide, on a 21" rim, with a 4.3" 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.