Is 275/55R20 bigger than 305/65R18?

275/55R20 is 1.70 inches shorter and 30 mm narrower. In overall diameter that is −5.06%, which exceeds the ±3% rule of thumb for a substitution.

How much will 275/55R20 throw off my speedometer?

Every reading scales by 0.94940. An indicated 60 mph becomes 57.0 mph, and every 100 miles on the odometer is really 94.9 miles.

How much does 275/55R20 change ground clearance over 305/65R18?

The axle centerline moves by 0.85 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 275/55R20 fit where 305/65R18 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.

305/65R18 vs 275/55R20

275/55R20 is −1.70″ in overall diameter and −30 mm in section width against 305/65R18 — a diameter change of −5.06%, which breaks the ±3% substitution rule. An indicated 60 mph would really be 57.0 mph.

Outside 3%

Every dimension

Difference between 305/65R18 and 275/55R20
Dimension	305/65R18 → 275/55R20	Change
Overall diameter	33.61″ → 31.91″	−1.70″ (−5.06%)
Section width	305 mm → 275 mm	−30 mm (−9.84%)
Sidewall height	198.3 mm → 151.3 mm	−47.0 mm
Rolling circumference	105.59″ → 100.25″	−5.34″
Revolutions per mile	600.1 → 632.0	+32.0
Axle height / clearance	16.81″ → 15.95″	−0.85″

305/65R18305 mm

275/55R20275 mm

305/65R18 33.61″275/55R20 31.91″

Which is taller, 305/65R18 or 275/55R20?

305/65R18 is the taller of the two, at 33.61″ against 31.91″. The gap is 1.70 inches, or 43.2 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. 305/65R18 is 305 mm wide with a 198.3 mm sidewall on a 18″ wheel; 275/55R20 is 275 mm wide with a 151.3 mm sidewall on a 20″ wheel.

What the speedometer will read

Indicated versus actual speed after the size change
Speedometer reads	Actual speed	Error
25 mph	23.7 mph	−1.3 mph
40 mph	38.0 mph	−2.0 mph
55 mph	52.2 mph	−2.8 mph
60 mph	57.0 mph	−3.0 mph
70 mph	66.5 mph	−3.5 mph
80 mph	76.0 mph	−4.0 mph

Fitting 275/55R20 in place of 305/65R18 multiplies every speed and distance reading by 0.94940. Over 10,000 indicated miles the odometer is out by −506 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 305/65R18 would need 3.54 to keep the same engine speed on 275/55R20. Work it out for your own axle ratio.

Full specification of each size

305/65R18

Derived dimensions for 305/65R18
Dimension	Metric	Imperial
Section width	305.0 mm	12.01″
Sidewall height	198.3 mm	7.81″
Wheel diameter	457.2 mm	18.0″
Overall diameter	853.7 mm	33.61″
Rolling circumference	2,682.0 mm	105.59″
Revolutions per mile	372.9 / km	600.1 / mile
Measuring rim (estimate)	229 mm	9″

275/55R20

Derived dimensions for 275/55R20
Dimension	Metric	Imperial
Section width	275.0 mm	10.83″
Sidewall height	151.3 mm	5.95″
Wheel diameter	508.0 mm	20.0″
Overall diameter	810.5 mm	31.91″
Rolling circumference	2,546.3 mm	100.25″
Revolutions per mile	392.7 / km	632.0 / mile
Measuring rim (estimate)	216 mm	8.5″

Rim widths differ too: 305/65R18 is designed for 8.5″–10″ and 275/55R20 for 8″–9.5″. 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 −5.06% exceeds the ±3% rule of thumb. Expect a speedometer error of about 3.0 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 275/55R20 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.