Question 1

Which tire is taller, 305/40 R22 or 285/60 R18?

Accepted Answer

305/40 R22 is taller by 3.6 mm (0.45%) compared with 285/60 R18.

Question 2

What is the diameter difference between 305/40 R22 and 285/60 R18?

Accepted Answer

305/40 R22 measures 803 mm overall diameter, 285/60 R18 measures 799 mm — a difference of 3.6 mm (0.45%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 305/40 R22 to 285/60 R18 changes the speedometer reading by -0.45%. At an indicated 100 km/h, your true speed becomes about 99.6 km/h.

Question 4

Is 285/60 R18 a safe replacement for 305/40 R22?

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	305/40 R22	285/60 R18	Difference
Overall diameter	802.8 mm	799.2 mm	-3.6 mm (-0.45%)
Sidewall height	122.0 mm	171.0 mm	+49.0 mm
Circumference	2.522 m	2.511 m	-11.3 mm
Revs / km	396.5	398.3	+1.8
Ground clearance	reference	-1.8 mm	-1.8 mm
Speedometer @ 100 km/h	100.0 km/h	99.6 km/h	-0.45 km/h

305/40 R22 vs 285/60 R18

Detailed comparison

Verdict: excellent

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