Question 1

Which tire is taller, 205/55 R16 or 225/40 R18?

Accepted Answer

225/40 R18 is taller by 5.3 mm (0.84%) compared with 205/55 R16.

Question 2

What is the diameter difference between 205/55 R16 and 225/40 R18?

Accepted Answer

205/55 R16 measures 632 mm overall diameter, 225/40 R18 measures 637 mm — a difference of 5.3 mm (0.84%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 205/55 R16 to 225/40 R18 changes the speedometer reading by +0.84%. At an indicated 100 km/h, your true speed becomes about 100.8 km/h.

Question 4

Is 225/40 R18 a safe replacement for 205/55 R16?

Accepted Answer

Within ±3% diameter — generally considered a safe replacement from a speedometer-error standpoint, provided wheel offset, rim width and clearance also match your vehicle.

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	205/55 R16	225/40 R18	Difference
Overall diameter	631.9 mm	637.2 mm	+5.3 mm (+0.84%)
Sidewall height	112.8 mm	90.0 mm	-22.8 mm
Circumference	1.985 m	2.002 m	+16.7 mm
Revs / km	503.7	499.5	-4.2
Ground clearance	reference	+2.7 mm	+2.7 mm
Speedometer @ 100 km/h	100.0 km/h	100.8 km/h	+0.84 km/h

205/55 R16 vs 225/40 R18

Detailed comparison

Verdict: excellent

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