Is 235/55R18 bigger than 235/65R17?

235/55R18 is 0.85 inches shorter and 0 mm wider. In overall diameter that is −2.93%, which stays inside the ±3% rule of thumb for a substitution.

How much will 235/55R18 throw off my speedometer?

Every reading scales by 0.97070. An indicated 60 mph becomes 58.2 mph, and every 100 miles on the odometer is really 97.1 miles.

How much does 235/55R18 change ground clearance over 235/65R17?

The axle centerline moves by 0.43 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 235/55R18 fit where 235/65R17 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.

235/65R17 vs 235/55R18

235/55R18 is −0.85″ in overall diameter and 0 mm in section width against 235/65R17 — a diameter change of −2.93%, which stays inside the ±3% substitution rule. An indicated 60 mph would really be 58.2 mph.

Within 3%

Every dimension

Difference between 235/65R17 and 235/55R18
Dimension	235/65R17 → 235/55R18	Change
Overall diameter	29.03″ → 28.18″	−0.85″ (−2.93%)
Section width	235 mm → 235 mm	0 mm (0.00%)
Sidewall height	152.8 mm → 129.3 mm	−23.5 mm
Rolling circumference	91.19″ → 88.52″	−2.67″
Revolutions per mile	694.8 → 715.8	+21.0
Axle height / clearance	14.51″ → 14.09″	−0.43″

235/65R17235 mm

235/55R18235 mm

235/65R17 29.03″235/55R18 28.18″

Which is taller, 235/65R17 or 235/55R18?

235/65R17 is the taller of the two, at 29.03″ against 28.18″. The gap is 0.85 inches, or 21.6 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. 235/65R17 is 235 mm wide with a 152.8 mm sidewall on a 17″ wheel; 235/55R18 is 235 mm wide with a 129.3 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	24.3 mph	−0.7 mph
40 mph	38.8 mph	−1.2 mph
55 mph	53.4 mph	−1.6 mph
60 mph	58.2 mph	−1.8 mph
70 mph	67.9 mph	−2.1 mph
80 mph	77.7 mph	−2.3 mph

Fitting 235/55R18 in place of 235/65R17 multiplies every speed and distance reading by 0.97070. Over 10,000 indicated miles the odometer is out by −293 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 235/65R17 would need 3.62 to keep the same engine speed on 235/55R18. Work it out for your own axle ratio.

Full specification of each size

235/65R17

Derived dimensions for 235/65R17
Dimension	Metric	Imperial
Section width	235.0 mm	9.25″
Sidewall height	152.8 mm	6.01″
Wheel diameter	431.8 mm	17.0″
Overall diameter	737.3 mm	29.03″
Rolling circumference	2,316.3 mm	91.19″
Revolutions per mile	431.7 / km	694.8 / mile
Measuring rim (estimate)	178 mm	7″

235/55R18

Derived dimensions for 235/55R18
Dimension	Metric	Imperial
Section width	235.0 mm	9.25″
Sidewall height	129.3 mm	5.09″
Wheel diameter	457.2 mm	18.0″
Overall diameter	715.7 mm	28.18″
Rolling circumference	2,248.4 mm	88.52″
Revolutions per mile	444.8 / km	715.8 / mile
Measuring rim (estimate)	191 mm	7.5″

Rim widths differ too: 235/65R17 is designed for 6.5″–8″ and 235/55R18 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 −2.93% 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 0 mm is a separate question and is the one that decides whether it rubs.

Neither figure says anything about physical fitment. Whether 235/55R18 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.