225/40R18 vs 225/50R16
225/50R16 has an overall diameter of 631.4 mm (24.86") compared with 637.2 mm (25.09") for 225/40R18, a difference of -5.8 mm (-0.91 %). The sidewall changes by +22.5 mm, section width by 0 mm and ride height by -2.9 mm. Using the ETRTO rolling circumference, 225/40R18 turns 515 times per kilometre and 225/50R16 turns 519 times. With 225/50R16 fitted to a car calibrated for 225/40R18, an indicated 100 km/h is a true 99.1 km/h, and the odometer records 100.9 km for every 100 km driven. This is within the commonly quoted 3 % tolerance.
Full comparison
| Measurement | 225/40R18 | 225/50R16 | Difference |
|---|---|---|---|
| Section width | 225 mm | 225 mm | 0 mm |
| Sidewall height | 90.0 mm | 112.5 mm | +22.5 mm |
| Rim diameter | 18" | 16" | -2.0" |
| Overall diameter | 637.2 mm (25.09") | 631.4 mm (24.86") | -5.8 mm (-0.91 %) |
| Static circumference | 2002 mm | 1984 mm | -18 mm |
| Rolling circumference (ETRTO) | 1943 mm | 1926 mm | -18 mm |
| Revs per km | 515 | 519 | +5 |
| Ride height | -2.9 mm | ||
| Inch equivalent | 25.1×8.9R18 | 24.9×8.9R16 | |
| Approved rim widths (approx.) | 7.5″–9″ | 6″–8″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 39.6 | 49.5 | 59.5 | 79.3 | 99.1 | 109.0 |
| Error (km/h) | -0.4 | -0.5 | -0.5 | -0.7 | -0.9 | -1.0 |
Assumes the speedo was accurate on 225/40R18. Odometer records 100.9 km for every 100 km actually driven.
- The 225/50R16 is 5.8 mm (0.91 %) smaller in diameter, so your speedo will read high and your odometer will over-record distance.
- Rim diameter changes from 18" to 16", so new wheels are required.
Is 225/50R16 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -5.8 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 smaller tyre makes the speedo read high: at an indicated 100 km/h you would be doing 99.1 km/h (over-read 0.9 km/h; ADR 18 allows up to 13.9 km/h at that speed). | OK |
| 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 225/50R16 instead of 225/40R18?
Overall diameter changes by -5.8 mm (-0.91 %). 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 225/50R16?
Your speedo will read high by about 0.91 %. At an indicated 100 km/h you would actually be travelling 99.1 km/h, and at 110 km/h about 109 km/h.
What is the rolling circumference of 225/50R16?
Static circumference is 1984 mm; the ETRTO rolling circumference under load is about 1926 mm, which is 519 revolutions per kilometre.
225/50R16 in inches
225/50R16 is approximately 24.9×8.9R16: 24.9" tall, 8.9" wide, on a 16" rim, with a 4.4" sidewall.
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