Is 225/45R18 bigger than 205/55R16?

225/45R18 is 1.09 inches taller and 20 mm wider. In overall diameter that is +4.40%, which exceeds the ±3% rule of thumb for a substitution.

How much will 225/45R18 throw off my speedometer?

Every reading scales by 1.04399. An indicated 60 mph becomes 62.6 mph, and every 100 miles on the odometer is really 104.4 miles.

How much does 225/45R18 change ground clearance over 205/55R16?

The axle centerline moves by 0.55 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 225/45R18 fit where 205/55R16 fits?

Nothing derived from the size code can answer that. Clearance depends on wheel offset, suspension travel and body dimensions that no tire size contains. This page tells you exactly what changes dimensionally, which is the part that can be calculated.

205/55R16 vs 225/45R18

225/45R18 is +1.09″ in overall diameter and +20 mm in section width against 205/55R16 — a diameter change of +4.40%, which breaks the ±3% substitution rule. An indicated 60 mph would really be 62.6 mph.

Outside 3%

Every dimension

Difference between 205/55R16 and 225/45R18
Dimension	205/55R16 → 225/45R18	Change
Overall diameter	24.88″ → 25.97″	+1.09″ (+4.40%)
Section width	205 mm → 225 mm	+20 mm (+9.76%)
Sidewall height	112.8 mm → 101.3 mm	−11.5 mm
Rolling circumference	78.16″ → 81.59″	+3.44″
Revolutions per mile	810.7 → 776.5	−34.2
Axle height / clearance	12.44″ → 12.99″	+0.55″

205/55R16205 mm

225/45R18225 mm

205/55R16 24.88″225/45R18 25.97″

Which is taller, 205/55R16 or 225/45R18?

225/45R18 is the taller of the two, at 25.97″ against 24.88″. The gap is 1.09 inches, or 27.8 mm.

Both are metric sizes, so neither states its height directly. Both figures below are derived from the section width, aspect ratio and wheel diameter in the code. 205/55R16 is 205 mm wide with a 112.8 mm sidewall on a 16″ wheel; 225/45R18 is 225 mm wide with a 101.3 mm sidewall on a 18″ wheel.

What the speedometer will read

Indicated versus actual speed after the size change
Speedometer reads	Actual speed	Error
25 mph	26.1 mph	+1.1 mph
40 mph	41.8 mph	+1.8 mph
55 mph	57.4 mph	+2.4 mph
60 mph	62.6 mph	+2.6 mph
70 mph	73.1 mph	+3.1 mph
80 mph	83.5 mph	+3.5 mph

Fitting 225/45R18 in place of 205/55R16 multiplies every speed and distance reading by 1.04399. Over 10,000 indicated miles the odometer is out by +440 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 205/55R16 would need 3.89 to keep the same engine speed on 225/45R18. Work it out for your own axle ratio.

Full specification of each size

205/55R16

Derived dimensions for 205/55R16
Dimension	Metric	Imperial
Section width	205.0 mm	8.07″
Sidewall height	112.8 mm	4.44″
Wheel diameter	406.4 mm	16.0″
Overall diameter	631.9 mm	24.88″
Rolling circumference	1,985.2 mm	78.16″
Revolutions per mile	503.7 / km	810.7 / mile
Measuring rim (estimate)	165 mm	6.5″

225/45R18

Derived dimensions for 225/45R18
Dimension	Metric	Imperial
Section width	225.0 mm	8.86″
Sidewall height	101.3 mm	3.99″
Wheel diameter	457.2 mm	18.0″
Overall diameter	659.7 mm	25.97″
Rolling circumference	2,072.5 mm	81.59″
Revolutions per mile	482.5 / km	776.5 / mile
Measuring rim (estimate)	191 mm	7.5″

Rim widths differ too: 205/55R16 is designed for 6″–7.5″ and 225/45R18 for 7″–8.5″. Their measuring rims differ, so the same wheel will not be ideal for both.

Can you swap one for the other?

Outside the ±3% rule

The diameter difference of +4.40% exceeds the ±3% rule of thumb. Expect a speedometer error of about 2.6 mph at an indicated 60, and an odometer that is off by the same proportion. It is a common swap even so — it just needs the instruments recalibrated to stay honest.

Neither figure says anything about physical fitment. Whether 225/45R18 clears the strut, the fender liner and the steering stops depends on wheel offset and suspension geometry that a size code does not encode. For maintenance rather than sizing, Car Basics covers tire care.