Question 1

Which tire is taller, 275/40 R18 or 285/30 R20?

Accepted Answer

285/30 R20 is taller by 1.8 mm (0.27%) compared with 275/40 R18.

Question 2

What is the diameter difference between 275/40 R18 and 285/30 R20?

Accepted Answer

275/40 R18 measures 677 mm overall diameter, 285/30 R20 measures 679 mm — a difference of 1.8 mm (0.27%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 275/40 R18 to 285/30 R20 changes the speedometer reading by +0.27%. At an indicated 100 km/h, your true speed becomes about 100.3 km/h.

Question 4

Is 285/30 R20 a safe replacement for 275/40 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	275/40 R18	285/30 R20	Difference
Overall diameter	677.2 mm	679.0 mm	+1.8 mm (+0.27%)
Sidewall height	110.0 mm	85.5 mm	-24.5 mm
Circumference	2.127 m	2.133 m	+5.7 mm
Revs / km	470.0	468.8	-1.2
Ground clearance	reference	+0.9 mm	+0.9 mm
Speedometer @ 100 km/h	100.0 km/h	100.3 km/h	+0.27 km/h

275/40 R18 vs 285/30 R20

Detailed comparison

Verdict: excellent

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