275/55R20 vs 275/65R18
275/65R18 has an overall diameter of 814.7 mm (32.07") compared with 810.5 mm (31.91") for 275/55R20, a difference of +4.2 mm (+0.52 %). The sidewall changes by +27.5 mm, section width by 0 mm and ride height by +2.1 mm. Using the ETRTO rolling circumference, 275/55R20 turns 405 times per kilometre and 275/65R18 turns 402 times. With 275/65R18 fitted to a car calibrated for 275/55R20, an indicated 100 km/h is a true 100.5 km/h, and the odometer records 99.5 km for every 100 km driven. This is within the commonly quoted 3 % tolerance.
Full comparison
| Measurement | 275/55R20 | 275/65R18 | Difference |
|---|---|---|---|
| Section width | 275 mm | 275 mm | 0 mm |
| Sidewall height | 151.3 mm | 178.8 mm | +27.5 mm |
| Rim diameter | 20" | 18" | -2.0" |
| Overall diameter | 810.5 mm (31.91") | 814.7 mm (32.07") | +4.2 mm (+0.52 %) |
| Static circumference | 2546 mm | 2559 mm | +13 mm |
| Rolling circumference (ETRTO) | 2472 mm | 2485 mm | +13 mm |
| Revs per km | 405 | 402 | -2 |
| Ride height | +2.1 mm | ||
| Inch equivalent | 31.9×10.8R20 | 32.1×10.8R18 | |
| Approved rim widths (approx.) | 7.5″–9.5″ | 7.5″–9″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 40.2 | 50.3 | 60.3 | 80.4 | 100.5 | 110.6 |
| Error (km/h) | +0.2 | +0.3 | +0.3 | +0.4 | +0.5 | +0.6 |
Assumes the speedo was accurate on 275/55R20. Odometer records 99.5 km for every 100 km actually driven.
- The 275/65R18 is 4.2 mm (0.52 %) larger in diameter, so your speedo will read low and your odometer will under-record distance.
- Rim diameter changes from 20" to 18", so new wheels are required.
Is 275/65R18 legal in Australia? Within limits, verify speedo
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of +4.2 mm is within the +15 mm / -26 mm allowance for a passenger car (MA/MB) (measured against the largest size the manufacturer designates for the model). | OK |
| Speedometer (ADR 18/03) A larger tyre makes the speedo read lower: at an indicated 100 km/h you would be doing 100.5 km/h. ADR 18 requires the speedo never to read below true speed, but factory speedos typically over-read by 2-6 %, so a 0.5 % 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 275/65R18 instead of 275/55R20?
Overall diameter changes by +4.2 mm (+0.52 %). That is within the usual 3 % rule of thumb, so it is generally an acceptable swap provided the load index and speed rating meet your tyre placard and the tyre suits your rim width. 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 275/65R18?
Your speedo will read low by about 0.52 %. At an indicated 100 km/h you would actually be travelling 100.5 km/h, and at 110 km/h about 110.6 km/h.
What is the rolling circumference of 275/65R18?
Static circumference is 2559 mm; the ETRTO rolling circumference under load is about 2485 mm, which is 402 revolutions per kilometre.
275/65R18 in inches
275/65R18 is approximately 32.1×10.8R18: 32.1" tall, 10.8" wide, on a 18" rim, with a 7.0" sidewall.
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