Question 1

Which tire is taller, 235/55 R18 or 245/40 R20?

Accepted Answer

235/55 R18 is taller by 11.7 mm (1.63%) compared with 245/40 R20.

Question 2

What is the diameter difference between 235/55 R18 and 245/40 R20?

Accepted Answer

235/55 R18 measures 716 mm overall diameter, 245/40 R20 measures 704 mm — a difference of 11.7 mm (1.63%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 235/55 R18 to 245/40 R20 changes the speedometer reading by -1.63%. At an indicated 100 km/h, your true speed becomes about 98.4 km/h.

Question 4

Is 245/40 R20 a safe replacement for 235/55 R18?

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	235/55 R18	245/40 R20	Difference
Overall diameter	715.7 mm	704.0 mm	-11.7 mm (-1.63%)
Sidewall height	129.3 mm	98.0 mm	-31.3 mm
Circumference	2.248 m	2.212 m	-36.8 mm
Revs / km	444.8	452.1	+7.4
Ground clearance	reference	-5.9 mm	-5.9 mm
Speedometer @ 100 km/h	100.0 km/h	98.4 km/h	-1.63 km/h

235/55 R18 vs 245/40 R20

Detailed comparison

Verdict: excellent

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