Question 1

Which tire is taller, 245/40 R20 or 225/45 R17?

Accepted Answer

245/40 R20 is taller by 69.7 mm (9.90%) compared with 225/45 R17.

Question 2

What is the diameter difference between 245/40 R20 and 225/45 R17?

Accepted Answer

245/40 R20 measures 704 mm overall diameter, 225/45 R17 measures 634 mm — a difference of 69.7 mm (9.90%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 245/40 R20 to 225/45 R17 changes the speedometer reading by -9.90%. At an indicated 100 km/h, your true speed becomes about 90.1 km/h.

Question 4

Is 225/45 R17 a safe replacement for 245/40 R20?

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	245/40 R20	225/45 R17	Difference
Overall diameter	704.0 mm	634.3 mm	-69.7 mm (-9.90%)
Sidewall height	98.0 mm	101.3 mm	+3.3 mm
Circumference	2.212 m	1.993 m	-219.0 mm
Revs / km	452.1	501.8	+49.7
Ground clearance	reference	-34.9 mm	-34.9 mm
Speedometer @ 100 km/h	100.0 km/h	90.1 km/h	-9.90 km/h

245/40 R20 vs 225/45 R17

Detailed comparison

Verdict: danger

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