245/45R18 vs 235/50R18
235/50R18 has an overall diameter of 692.2 mm (27.25") compared with 677.7 mm (26.68") for 245/45R18, a difference of +14.5 mm (+2.14 %). The sidewall changes by +7.3 mm, section width by -10 mm and ride height by +7.3 mm. Using the ETRTO rolling circumference, 245/45R18 turns 484 times per kilometre and 235/50R18 turns 474 times. With 235/50R18 fitted to a car calibrated for 245/45R18, 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 | 245/45R18 | 235/50R18 | Difference |
|---|---|---|---|
| Section width | 245 mm | 235 mm | -10 mm |
| Sidewall height | 110.3 mm | 117.5 mm | +7.3 mm |
| Rim diameter | 18" | 18" | 0.0" |
| Overall diameter | 677.7 mm (26.68") | 692.2 mm (27.25") | +14.5 mm (+2.14 %) |
| Static circumference | 2129 mm | 2175 mm | +46 mm |
| Rolling circumference (ETRTO) | 2067 mm | 2111 mm | +44 mm |
| Revs per km | 484 | 474 | -10 |
| Ride height | +7.3 mm | ||
| Inch equivalent | 26.7×9.6R18 | 27.3×9.3R18 | |
| Approved rim widths (approx.) | 7.5″–9″ | 6.5″–8.5″ |
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 245/45R18. Odometer records 97.9 km for every 100 km actually driven.
- The 235/50R18 is 14.5 mm (2.14 %) larger in diameter, so your speedo will read low and your odometer will under-record distance.
- Section width changes by -10 mm. Check guard, strut and inner-arch clearance and that the rim width is within the approved range for the new tyre.
Is 235/50R18 legal in Australia? Within limits, verify speedo
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of +14.5 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 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 235/50R18 instead of 245/45R18?
Overall diameter changes by +14.5 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 235/50R18?
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 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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