Question 1

Which tire is taller, 245/40 R17 or 225/45 R18?

Accepted Answer

225/45 R18 is taller by 31.9 mm (5.08%) compared with 245/40 R17.

Question 2

What is the diameter difference between 245/40 R17 and 225/45 R18?

Accepted Answer

245/40 R17 measures 628 mm overall diameter, 225/45 R18 measures 660 mm — a difference of 31.9 mm (5.08%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 245/40 R17 to 225/45 R18 changes the speedometer reading by +5.08%. At an indicated 100 km/h, your true speed becomes about 105.1 km/h.

Question 4

Is 225/45 R18 a safe replacement for 245/40 R17?

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	245/40 R17	225/45 R18	Difference
Overall diameter	627.8 mm	659.7 mm	+31.9 mm (+5.08%)
Sidewall height	98.0 mm	101.3 mm	+3.3 mm
Circumference	1.972 m	2.073 m	+100.2 mm
Revs / km	507.0	482.5	-24.5
Ground clearance	reference	+16.0 mm	+16.0 mm
Speedometer @ 100 km/h	100.0 km/h	105.1 km/h	+5.08 km/h

245/40 R17 vs 225/45 R18

Detailed comparison

Verdict: danger

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