Is 225/45R18 bigger than 215/55R17?

225/45R18 is 0.34 inches shorter and 10 mm wider. In overall diameter that is −1.29%, which stays inside the ±3% rule of thumb for a substitution.

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

Every reading scales by 0.98713. An indicated 60 mph becomes 59.2 mph, and every 100 miles on the odometer is really 98.7 miles.

How much does 225/45R18 change ground clearance over 215/55R17?

The axle centerline moves by 0.17 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 225/45R18 fit where 215/55R17 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.

215/55R17 vs 225/45R18

225/45R18 is −0.34″ in overall diameter and +10 mm in section width against 215/55R17 — a diameter change of −1.29%, which stays inside the ±3% substitution rule. An indicated 60 mph would really be 59.2 mph.

Within 3%

Every dimension

Difference between 215/55R17 and 225/45R18
Dimension	215/55R17 → 225/45R18	Change
Overall diameter	26.31″ → 25.97″	−0.34″ (−1.29%)
Section width	215 mm → 225 mm	+10 mm (+4.65%)
Sidewall height	118.3 mm → 101.3 mm	−17.0 mm
Rolling circumference	82.66″ → 81.59″	−1.06″
Revolutions per mile	766.5 → 776.5	+10.0
Axle height / clearance	13.16″ → 12.99″	−0.17″

215/55R17215 mm

225/45R18225 mm

215/55R17 26.31″225/45R18 25.97″

Which is taller, 215/55R17 or 225/45R18?

215/55R17 is the taller of the two, at 26.31″ against 25.97″. The gap is 0.34 inches, or 8.6 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. 215/55R17 is 215 mm wide with a 118.3 mm sidewall on a 17″ 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	24.7 mph	−0.3 mph
40 mph	39.5 mph	−0.5 mph
55 mph	54.3 mph	−0.7 mph
60 mph	59.2 mph	−0.8 mph
70 mph	69.1 mph	−0.9 mph
80 mph	79.0 mph	−1.0 mph

Fitting 225/45R18 in place of 215/55R17 multiplies every speed and distance reading by 0.98713. Over 10,000 indicated miles the odometer is out by −129 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 215/55R17 would need 3.68 to keep the same engine speed on 225/45R18. Work it out for your own axle ratio.

Full specification of each size

215/55R17

Derived dimensions for 215/55R17
Dimension	Metric	Imperial
Section width	215.0 mm	8.46″
Sidewall height	118.3 mm	4.66″
Wheel diameter	431.8 mm	17.0″
Overall diameter	668.3 mm	26.31″
Rolling circumference	2,099.5 mm	82.66″
Revolutions per mile	476.3 / km	766.5 / mile
Measuring rim (estimate)	178 mm	7″

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: 215/55R17 is designed for 6.5″–8″ 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?

Inside the ±3% rule

The diameter difference of −1.29% is within the ±3% that fitters use as the boundary for a substitution, so the speedometer, odometer and wheel-speed-derived systems stay close enough to calibration. The section width difference of +10 mm is a separate question and is the one that decides whether it rubs.

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.