225/45R18 vs 235/40R19
235/40R19 has an overall diameter of 670.6 mm (26.40") compared with 659.7 mm (25.97") for 225/45R18, a difference of +10.9 mm (+1.65 %). The sidewall changes by -7.2 mm, section width by +10 mm and ride height by +5.4 mm. Using the ETRTO rolling circumference, 225/45R18 turns 497 times per kilometre and 235/40R19 turns 489 times. With 235/40R19 fitted to a car calibrated for 225/45R18, an indicated 100 km/h is a true 101.7 km/h, and the odometer records 98.4 km for every 100 km driven. This is within the commonly quoted 3 % tolerance.
Full comparison
| Measurement | 225/45R18 | 235/40R19 | Difference |
|---|---|---|---|
| Section width | 225 mm | 235 mm | +10 mm |
| Sidewall height | 101.3 mm | 94.0 mm | -7.2 mm |
| Rim diameter | 18" | 19" | +1.0" |
| Overall diameter | 659.7 mm (25.97") | 670.6 mm (26.40") | +10.9 mm (+1.65 %) |
| Static circumference | 2073 mm | 2107 mm | +34 mm |
| Rolling circumference (ETRTO) | 2012 mm | 2045 mm | +33 mm |
| Revs per km | 497 | 489 | -8 |
| Ride height | +5.4 mm | ||
| Inch equivalent | 26×8.9R18 | 26.4×9.3R19 | |
| Approved rim widths (approx.) | 7″–8.5″ | 8″–9.5″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 40.7 | 50.8 | 61.0 | 81.3 | 101.7 | 111.8 |
| Error (km/h) | +0.7 | +0.8 | +1.0 | +1.3 | +1.7 | +1.8 |
Assumes the speedo was accurate on 225/45R18. Odometer records 98.4 km for every 100 km actually driven.
- The 235/40R19 is 10.9 mm (1.65 %) 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.
- Rim diameter changes from 18" to 19", so new wheels are required.
Is 235/40R19 legal in Australia? Within limits, verify speedo
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of +10.9 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 101.7 km/h. ADR 18 requires the speedo never to read below true speed, but factory speedos typically over-read by 2-6 %, so a 1.7 % 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 235/40R19 instead of 225/45R18?
Overall diameter changes by +10.9 mm (+1.65 %). 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 235/40R19?
Your speedo will read low by about 1.65 %. At an indicated 100 km/h you would actually be travelling 101.7 km/h, and at 110 km/h about 111.8 km/h.
What is the rolling circumference of 235/40R19?
Static circumference is 2107 mm; the ETRTO rolling circumference under load is about 2045 mm, which is 489 revolutions per kilometre.
235/40R19 in inches
235/40R19 is approximately 26.4×9.3R19: 26.4" tall, 9.3" wide, on a 19" rim, with a 3.7" sidewall.
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