275/50R20 vs 305/40R22
305/40R22 has an overall diameter of 802.8 mm (31.61") compared with 783.0 mm (30.83") for 275/50R20, a difference of +19.8 mm (+2.53 %). The sidewall changes by -15.5 mm, section width by +30 mm and ride height by +9.9 mm. Using the ETRTO rolling circumference, 275/50R20 turns 419 times per kilometre and 305/40R22 turns 408 times. With 305/40R22 fitted to a car calibrated for 275/50R20, an indicated 100 km/h is a true 102.5 km/h, and the odometer records 97.5 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 275/50R20 | 305/40R22 | Difference |
|---|---|---|---|
| Section width | 275 mm | 305 mm | +30 mm |
| Sidewall height | 137.5 mm | 122.0 mm | -15.5 mm |
| Rim diameter | 20" | 22" | +2.0" |
| Overall diameter | 783.0 mm (30.83") | 802.8 mm (31.61") | +19.8 mm (+2.53 %) |
| Static circumference | 2460 mm | 2522 mm | +62 mm |
| Rolling circumference (ETRTO) | 2388 mm | 2449 mm | +60 mm |
| Revs per km | 419 | 408 | -10 |
| Ride height | +9.9 mm | ||
| Inch equivalent | 30.8×10.8R20 | 31.6x12R22 | |
| Approved rim widths (approx.) | 7.5″–9.5″ | 10.5″–12″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 41.0 | 51.3 | 61.5 | 82.0 | 102.5 | 112.8 |
| Error (km/h) | +1.0 | +1.3 | +1.5 | +2.0 | +2.5 | +2.8 |
Assumes the speedo was accurate on 275/50R20. Odometer records 97.5 km for every 100 km actually driven.
- The 305/40R22 is 19.8 mm (2.53 %) larger in diameter, so your speedo will read low and your odometer will under-record distance.
- Section width changes by +30 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 20" to 22", so new wheels are required.
Is 305/40R22 legal in Australia? Needs certification
| Overall diameter (VSB 14 LS 4.2.4) 305/40R22 is 19.8 mm larger than 275/50R20; 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 102.5 km/h. ADR 18 requires the speedo never to read below true speed, but factory speedos typically over-read by 2-6 %, so a 2.5 % 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 305/40R22 instead of 275/50R20?
Overall diameter changes by +19.8 mm (+2.53 %). 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 305/40R22?
Your speedo will read low by about 2.53 %. At an indicated 100 km/h you would actually be travelling 102.5 km/h, and at 110 km/h about 112.8 km/h.
What is the rolling circumference of 305/40R22?
Static circumference is 2522 mm; the ETRTO rolling circumference under load is about 2449 mm, which is 408 revolutions per kilometre.
305/40R22 in inches
305/40R22 is approximately 31.6x12R22: 31.6" tall, 12.0" wide, on a 22" rim, with a 4.8" sidewall.
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