Question 1

Which tire is taller, 235/45 R18 or 225/45 R17?

Accepted Answer

235/45 R18 is taller by 34.4 mm (5.14%) compared with 225/45 R17.

Question 2

What is the diameter difference between 235/45 R18 and 225/45 R17?

Accepted Answer

235/45 R18 measures 669 mm overall diameter, 225/45 R17 measures 634 mm — a difference of 34.4 mm (5.14%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 235/45 R18 to 225/45 R17 changes the speedometer reading by -5.14%. At an indicated 100 km/h, your true speed becomes about 94.9 km/h.

Question 4

Is 225/45 R17 a safe replacement for 235/45 R18?

Accepted Answer

Over 5% diameter difference. This is outside the common safe range and can affect speedometer accuracy, ABS calibration and gearing. Not recommended without professional review.

Question 5

What else should I check before changing tire size?

Accepted Answer

Beyond diameter and speedometer error, verify wheel offset (ET), rim width, hub bore, load index, brake and suspension clearance, and your vehicle's manufacturer specification. Local regulations may also limit how far you can deviate from OEM.

Metric	235/45 R18	225/45 R17	Difference
Overall diameter	668.7 mm	634.3 mm	-34.4 mm (-5.14%)
Sidewall height	105.8 mm	101.3 mm	-4.5 mm
Circumference	2.101 m	1.993 m	-108.1 mm
Revs / km	476.0	501.8	+25.8
Ground clearance	reference	-17.2 mm	-17.2 mm
Speedometer @ 100 km/h	100.0 km/h	94.9 km/h	-5.14 km/h

235/45 R18 vs 225/45 R17

Detailed comparison

Verdict: danger

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