Question 1

Which tire is taller, 255/40 R19 or 285/30 R20?

Accepted Answer

255/40 R19 is taller by 7.6 mm (1.11%) compared with 285/30 R20.

Question 2

What is the diameter difference between 255/40 R19 and 285/30 R20?

Accepted Answer

255/40 R19 measures 687 mm overall diameter, 285/30 R20 measures 679 mm — a difference of 7.6 mm (1.11%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 255/40 R19 to 285/30 R20 changes the speedometer reading by -1.11%. At an indicated 100 km/h, your true speed becomes about 98.9 km/h.

Question 4

Is 285/30 R20 a safe replacement for 255/40 R19?

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	255/40 R19	285/30 R20	Difference
Overall diameter	686.6 mm	679.0 mm	-7.6 mm (-1.11%)
Sidewall height	102.0 mm	85.5 mm	-16.5 mm
Circumference	2.157 m	2.133 m	-23.9 mm
Revs / km	463.6	468.8	+5.2
Ground clearance	reference	-3.8 mm	-3.8 mm
Speedometer @ 100 km/h	100.0 km/h	98.9 km/h	-1.11 km/h

255/40 R19 vs 285/30 R20

Detailed comparison

Verdict: excellent

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