Question 1

Which tire is taller, 245/40 R19 or 305/30 R20?

Accepted Answer

305/30 R20 is taller by 12.4 mm (1.83%) compared with 245/40 R19.

Question 2

What is the diameter difference between 245/40 R19 and 305/30 R20?

Accepted Answer

245/40 R19 measures 679 mm overall diameter, 305/30 R20 measures 691 mm — a difference of 12.4 mm (1.83%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 245/40 R19 to 305/30 R20 changes the speedometer reading by +1.83%. At an indicated 100 km/h, your true speed becomes about 101.8 km/h.

Question 4

Is 305/30 R20 a safe replacement for 245/40 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	245/40 R19	305/30 R20	Difference
Overall diameter	678.6 mm	691.0 mm	+12.4 mm (+1.83%)
Sidewall height	98.0 mm	91.5 mm	-6.5 mm
Circumference	2.132 m	2.171 m	+39.0 mm
Revs / km	469.1	460.7	-8.4
Ground clearance	reference	+6.2 mm	+6.2 mm
Speedometer @ 100 km/h	100.0 km/h	101.8 km/h	+1.83 km/h

245/40 R19 vs 305/30 R20

Detailed comparison

Verdict: excellent

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