Is 235/50R18 bigger than 225/50R17?

235/50R18 is 1.39 inches taller and 10 mm wider. In overall diameter that is +5.39%, which exceeds the ±3% rule of thumb for a substitution.

How much will 235/50R18 throw off my speedometer?

Every reading scales by 1.05390. An indicated 60 mph becomes 63.2 mph, and every 100 miles on the odometer is really 105.4 miles.

How much does 235/50R18 change ground clearance over 225/50R17?

The axle centerline moves by 0.70 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 235/50R18 fit where 225/50R17 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.

225/50R17 vs 235/50R18

235/50R18 is +1.39″ in overall diameter and +10 mm in section width against 225/50R17 — a diameter change of +5.39%, which breaks the ±3% substitution rule. An indicated 60 mph would really be 63.2 mph.

Outside 3%

Every dimension

Difference between 225/50R17 and 235/50R18
Dimension	225/50R17 → 235/50R18	Change
Overall diameter	25.86″ → 27.25″	+1.39″ (+5.39%)
Section width	225 mm → 235 mm	+10 mm (+4.44%)
Sidewall height	112.5 mm → 117.5 mm	+5.0 mm
Rolling circumference	81.24″ → 85.61″	+4.38″
Revolutions per mile	779.9 → 740.1	−39.9
Axle height / clearance	12.93″ → 13.63″	+0.70″

225/50R17225 mm

235/50R18235 mm

225/50R17 25.86″235/50R18 27.25″

Which is taller, 225/50R17 or 235/50R18?

235/50R18 is the taller of the two, at 27.25″ against 25.86″. The gap is 1.39 inches, or 35.4 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. 225/50R17 is 225 mm wide with a 112.5 mm sidewall on a 17″ wheel; 235/50R18 is 235 mm wide with a 117.5 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	26.3 mph	+1.3 mph
40 mph	42.2 mph	+2.2 mph
55 mph	58.0 mph	+3.0 mph
60 mph	63.2 mph	+3.2 mph
70 mph	73.8 mph	+3.8 mph
80 mph	84.3 mph	+4.3 mph

Fitting 235/50R18 in place of 225/50R17 multiplies every speed and distance reading by 1.05390. Over 10,000 indicated miles the odometer is out by +539 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 225/50R17 would need 3.93 to keep the same engine speed on 235/50R18. Work it out for your own axle ratio.

Full specification of each size

225/50R17

Derived dimensions for 225/50R17
Dimension	Metric	Imperial
Section width	225.0 mm	8.86″
Sidewall height	112.5 mm	4.43″
Wheel diameter	431.8 mm	17.0″
Overall diameter	656.8 mm	25.86″
Rolling circumference	2,063.4 mm	81.24″
Revolutions per mile	484.6 / km	779.9 / mile
Measuring rim (estimate)	191 mm	7.5″

235/50R18

Derived dimensions for 235/50R18
Dimension	Metric	Imperial
Section width	235.0 mm	9.25″
Sidewall height	117.5 mm	4.63″
Wheel diameter	457.2 mm	18.0″
Overall diameter	692.2 mm	27.25″
Rolling circumference	2,174.6 mm	85.61″
Revolutions per mile	459.9 / km	740.1 / mile
Measuring rim (estimate)	191 mm	7.5″

Rim widths differ too: 225/50R17 is designed for 7″–8.5″ and 235/50R18 for 7″–8.5″. They share a measuring rim, so one wheel can usually carry either.

Can you swap one for the other?

Outside the ±3% rule

The diameter difference of +5.39% exceeds the ±3% rule of thumb. Expect a speedometer error of about 3.2 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 235/50R18 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.