Question 1

Which tire is taller, 255/40 R19 or 245/45 R20?

Accepted Answer

245/45 R20 is taller by 41.9 mm (6.10%) compared with 255/40 R19.

Question 2

What is the diameter difference between 255/40 R19 and 245/45 R20?

Accepted Answer

255/40 R19 measures 687 mm overall diameter, 245/45 R20 measures 729 mm — a difference of 41.9 mm (6.10%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 255/40 R19 to 245/45 R20 changes the speedometer reading by +6.10%. At an indicated 100 km/h, your true speed becomes about 106.1 km/h.

Question 4

Is 245/45 R20 a safe replacement for 255/40 R19?

Accepted Answer

Over 5% diameter difference. This is outside the common safe range and can affect speedometer accuracy, ABS calibration and gearing. Not recommended without professional review.

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	245/45 R20	Difference
Overall diameter	686.6 mm	728.5 mm	+41.9 mm (+6.10%)
Sidewall height	102.0 mm	110.3 mm	+8.3 mm
Circumference	2.157 m	2.289 m	+131.6 mm
Revs / km	463.6	436.9	-26.7
Ground clearance	reference	+21.0 mm	+21.0 mm
Speedometer @ 100 km/h	100.0 km/h	106.1 km/h	+6.10 km/h

255/40 R19 vs 245/45 R20

Detailed comparison

Verdict: danger

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