Question 1

Which tire is taller, 255/50 R19 or 245/40 R20?

Accepted Answer

255/50 R19 is taller by 33.6 mm (4.56%) compared with 245/40 R20.

Question 2

What is the diameter difference between 255/50 R19 and 245/40 R20?

Accepted Answer

255/50 R19 measures 738 mm overall diameter, 245/40 R20 measures 704 mm — a difference of 33.6 mm (4.56%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 255/50 R19 to 245/40 R20 changes the speedometer reading by -4.56%. At an indicated 100 km/h, your true speed becomes about 95.4 km/h.

Question 4

Is 245/40 R20 a safe replacement for 255/50 R19?

Accepted Answer

Between 3% and 5% diameter difference — speedometer drift is noticeable. Consider recalibration and confirm clearance with your vehicle's manufacturer specification.

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	255/50 R19	245/40 R20	Difference
Overall diameter	737.6 mm	704.0 mm	-33.6 mm (-4.56%)
Sidewall height	127.5 mm	98.0 mm	-29.5 mm
Circumference	2.317 m	2.212 m	-105.6 mm
Revs / km	431.5	452.1	+20.6
Ground clearance	reference	-16.8 mm	-16.8 mm
Speedometer @ 100 km/h	100.0 km/h	95.4 km/h	-4.56 km/h

255/50 R19 vs 245/40 R20

Detailed comparison

Verdict: warning

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