Question 1

Which tire is taller, 275/40 R19 or 305/30 R20?

Accepted Answer

275/40 R19 is taller by 11.6 mm (1.65%) compared with 305/30 R20.

Question 2

What is the diameter difference between 275/40 R19 and 305/30 R20?

Accepted Answer

275/40 R19 measures 703 mm overall diameter, 305/30 R20 measures 691 mm — a difference of 11.6 mm (1.65%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 275/40 R19 to 305/30 R20 changes the speedometer reading by -1.65%. At an indicated 100 km/h, your true speed becomes about 98.3 km/h.

Question 4

Is 305/30 R20 a safe replacement for 275/40 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	275/40 R19	305/30 R20	Difference
Overall diameter	702.6 mm	691.0 mm	-11.6 mm (-1.65%)
Sidewall height	110.0 mm	91.5 mm	-18.5 mm
Circumference	2.207 m	2.171 m	-36.4 mm
Revs / km	453.0	460.7	+7.6
Ground clearance	reference	-5.8 mm	-5.8 mm
Speedometer @ 100 km/h	100.0 km/h	98.3 km/h	-1.65 km/h

275/40 R19 vs 305/30 R20

Detailed comparison

Verdict: excellent

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