Is 275/60R20 bigger than 305/65R18?

275/60R20 is 0.62 inches shorter and 30 mm narrower. In overall diameter that is −1.84%, which stays inside the ±3% rule of thumb for a substitution.

How much will 275/60R20 throw off my speedometer?

Every reading scales by 0.98161. An indicated 60 mph becomes 58.9 mph, and every 100 miles on the odometer is really 98.2 miles.

How much does 275/60R20 change ground clearance over 305/65R18?

The axle centerline moves by 0.31 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 275/60R20 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 275/60R20

275/60R20 is −0.62″ in overall diameter and −30 mm in section width against 305/65R18 — a diameter change of −1.84%, which stays inside the ±3% substitution rule. An indicated 60 mph would really be 58.9 mph.

Within 3%

Every dimension

Difference between 305/65R18 and 275/60R20
Dimension	305/65R18 → 275/60R20	Change
Overall diameter	33.61″ → 32.99″	−0.62″ (−1.84%)
Section width	305 mm → 275 mm	−30 mm (−9.84%)
Sidewall height	198.3 mm → 165.0 mm	−33.3 mm
Rolling circumference	105.59″ → 103.65″	−1.94″
Revolutions per mile	600.1 → 611.3	+11.2
Axle height / clearance	16.81″ → 16.50″	−0.31″

305/65R18305 mm

275/60R20275 mm

305/65R18 33.61″275/60R20 32.99″

Which is taller, 305/65R18 or 275/60R20?

305/65R18 is the taller of the two, at 33.61″ against 32.99″. The gap is 0.62 inches, or 15.7 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; 275/60R20 is 275 mm wide with a 165.0 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	24.5 mph	−0.5 mph
40 mph	39.3 mph	−0.7 mph
55 mph	54.0 mph	−1.0 mph
60 mph	58.9 mph	−1.1 mph
70 mph	68.7 mph	−1.3 mph
80 mph	78.5 mph	−1.5 mph

Fitting 275/60R20 in place of 305/65R18 multiplies every speed and distance reading by 0.98161. Over 10,000 indicated miles the odometer is out by −184 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 305/65R18 would need 3.66 to keep the same engine speed on 275/60R20. 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″

275/60R20

Derived dimensions for 275/60R20
Dimension	Metric	Imperial
Section width	275.0 mm	10.83″
Sidewall height	165.0 mm	6.50″
Wheel diameter	508.0 mm	20.0″
Overall diameter	838.0 mm	32.99″
Rolling circumference	2,632.7 mm	103.65″
Revolutions per mile	379.8 / km	611.3 / mile
Measuring rim (estimate)	216 mm	8.5″

Rim widths differ too: 305/65R18 is designed for 8.5″–10″ and 275/60R20 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?

Inside the ±3% rule

The diameter difference of −1.84% 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 −30 mm is a separate question and is the one that decides whether it rubs.

Neither figure says anything about physical fitment. Whether 275/60R20 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.