Question 1

Which tire is taller, 235/40 R18 or 225/45 R17?

Accepted Answer

235/40 R18 is taller by 10.9 mm (1.69%) compared with 225/45 R17.

Question 2

What is the diameter difference between 235/40 R18 and 225/45 R17?

Accepted Answer

235/40 R18 measures 645 mm overall diameter, 225/45 R17 measures 634 mm — a difference of 10.9 mm (1.69%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 235/40 R18 to 225/45 R17 changes the speedometer reading by -1.69%. At an indicated 100 km/h, your true speed becomes about 98.3 km/h.

Question 4

Is 225/45 R17 a safe replacement for 235/40 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	235/40 R18	225/45 R17	Difference
Overall diameter	645.2 mm	634.3 mm	-10.9 mm (-1.69%)
Sidewall height	94.0 mm	101.3 mm	+7.3 mm
Circumference	2.027 m	1.993 m	-34.2 mm
Revs / km	493.4	501.8	+8.5
Ground clearance	reference	-5.5 mm	-5.5 mm
Speedometer @ 100 km/h	100.0 km/h	98.3 km/h	-1.69 km/h

235/40 R18 vs 225/45 R17

Detailed comparison

Verdict: excellent

Related comparisons

Frequently asked questions