Is 235/45R18 bigger than 225/45R18?

235/45R18 is 0.35 inches taller and 10 mm wider. In overall diameter that is +1.36%, which stays inside the ±3% rule of thumb for a substitution.

How much will 235/45R18 throw off my speedometer?

Every reading scales by 1.01364. An indicated 60 mph becomes 60.8 mph, and every 100 miles on the odometer is really 101.4 miles.

How much does 235/45R18 change ground clearance over 225/45R18?

The axle centerline moves by 0.18 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 235/45R18 fit where 225/45R18 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.

225/45R18 vs 235/45R18

235/45R18 is +0.35″ in overall diameter and +10 mm in section width against 225/45R18 — a diameter change of +1.36%, which stays inside the ±3% substitution rule. An indicated 60 mph would really be 60.8 mph.

Within 3%

Every dimension

Difference between 225/45R18 and 235/45R18
Dimension	225/45R18 → 235/45R18	Change
Overall diameter	25.97″ → 26.33″	+0.35″ (+1.36%)
Section width	225 mm → 235 mm	+10 mm (+4.44%)
Sidewall height	101.3 mm → 105.8 mm	+4.5 mm
Rolling circumference	81.59″ → 82.71″	+1.11″
Revolutions per mile	776.5 → 766.1	−10.5
Axle height / clearance	12.99″ → 13.16″	+0.18″

225/45R18225 mm

235/45R18235 mm

225/45R18 25.97″235/45R18 26.33″

Which is taller, 225/45R18 or 235/45R18?

235/45R18 is the taller of the two, at 26.33″ against 25.97″. The gap is 0.35 inches, or 9.0 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. 225/45R18 is 225 mm wide with a 101.3 mm sidewall on a 18″ wheel; 235/45R18 is 235 mm wide with a 105.8 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	25.3 mph	+0.3 mph
40 mph	40.5 mph	+0.5 mph
55 mph	55.8 mph	+0.8 mph
60 mph	60.8 mph	+0.8 mph
70 mph	71.0 mph	+1.0 mph
80 mph	81.1 mph	+1.1 mph

Fitting 235/45R18 in place of 225/45R18 multiplies every speed and distance reading by 1.01364. Over 10,000 indicated miles the odometer is out by +136 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 225/45R18 would need 3.78 to keep the same engine speed on 235/45R18. Work it out for your own axle ratio.

Full specification of each size

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″

235/45R18

Derived dimensions for 235/45R18
Dimension	Metric	Imperial
Section width	235.0 mm	9.25″
Sidewall height	105.8 mm	4.16″
Wheel diameter	457.2 mm	18.0″
Overall diameter	668.7 mm	26.33″
Rolling circumference	2,100.8 mm	82.71″
Revolutions per mile	476.0 / km	766.1 / mile
Measuring rim (estimate)	203 mm	8″

Rim widths differ too: 225/45R18 is designed for 7″–8.5″ and 235/45R18 for 7.5″–9″. 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 +1.36% 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 235/45R18 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.