235/55R17 vs 235/50R18
235/50R18 has an overall diameter of 692.2 mm (27.25") compared with 690.3 mm (27.18") for 235/55R17, a difference of +1.9 mm (+0.28 %). The sidewall changes by -11.7 mm, section width by 0 mm and ride height by +1.0 mm. Using the ETRTO rolling circumference, 235/55R17 turns 475 times per kilometre and 235/50R18 turns 474 times. With 235/50R18 fitted to a car calibrated for 235/55R17, an indicated 100 km/h is a true 100.3 km/h, and the odometer records 99.7 km for every 100 km driven. This is within the commonly quoted 3 % tolerance.
Full comparison
| Measurement | 235/55R17 | 235/50R18 | Difference |
|---|---|---|---|
| Section width | 235 mm | 235 mm | 0 mm |
| Sidewall height | 129.3 mm | 117.5 mm | -11.7 mm |
| Rim diameter | 17" | 18" | +1.0" |
| Overall diameter | 690.3 mm (27.18") | 692.2 mm (27.25") | +1.9 mm (+0.28 %) |
| Static circumference | 2169 mm | 2175 mm | +6 mm |
| Rolling circumference (ETRTO) | 2105 mm | 2111 mm | +6 mm |
| Revs per km | 475 | 474 | -1 |
| Ride height | +1.0 mm | ||
| Inch equivalent | 27.2×9.3R17 | 27.3×9.3R18 | |
| Approved rim widths (approx.) | 6.5″–8.5″ | 6.5″–8.5″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 40.1 | 50.1 | 60.2 | 80.2 | 100.3 | 110.3 |
| Error (km/h) | +0.1 | +0.1 | +0.2 | +0.2 | +0.3 | +0.3 |
Assumes the speedo was accurate on 235/55R17. Odometer records 99.7 km for every 100 km actually driven.
- The 235/50R18 is 1.9 mm (0.28 %) larger in diameter, so your speedo will read low and your odometer will under-record distance.
- Rim diameter changes from 17" to 18", so new wheels are required.
Is 235/50R18 legal in Australia? Within limits, verify speedo
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of +1.9 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 100.3 km/h. ADR 18 requires the speedo never to read below true speed, but factory speedos typically over-read by 2-6 %, so a 0.3 % 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 235/50R18 instead of 235/55R17?
Overall diameter changes by +1.9 mm (+0.28 %). 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 235/50R18?
Your speedo will read low by about 0.28 %. At an indicated 100 km/h you would actually be travelling 100.3 km/h, and at 110 km/h about 110.3 km/h.
What is the rolling circumference of 235/50R18?
Static circumference is 2175 mm; the ETRTO rolling circumference under load is about 2111 mm, which is 474 revolutions per kilometre.
235/50R18 in inches
235/50R18 is approximately 27.3×9.3R18: 27.3" tall, 9.3" wide, on a 18" rim, with a 4.6" sidewall.
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