Question 1

Which tire is taller, 225/50 R18 or 285/30 R20?

Accepted Answer

225/50 R18 is taller by 3.2 mm (0.47%) compared with 285/30 R20.

Question 2

What is the diameter difference between 225/50 R18 and 285/30 R20?

Accepted Answer

225/50 R18 measures 682 mm overall diameter, 285/30 R20 measures 679 mm — a difference of 3.2 mm (0.47%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 225/50 R18 to 285/30 R20 changes the speedometer reading by -0.47%. At an indicated 100 km/h, your true speed becomes about 99.5 km/h.

Question 4

Is 285/30 R20 a safe replacement for 225/50 R18?

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	225/50 R18	285/30 R20	Difference
Overall diameter	682.2 mm	679.0 mm	-3.2 mm (-0.47%)
Sidewall height	112.5 mm	85.5 mm	-27.0 mm
Circumference	2.143 m	2.133 m	-10.1 mm
Revs / km	466.6	468.8	+2.2
Ground clearance	reference	-1.6 mm	-1.6 mm
Speedometer @ 100 km/h	100.0 km/h	99.5 km/h	-0.47 km/h

225/50 R18 vs 285/30 R20

Detailed comparison

Verdict: excellent

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