Is 305/55R20 bigger than 265/70R18?

305/55R20 is 0.60 inches taller and 40 mm wider. In overall diameter that is +1.85%, which stays inside the ±3% rule of thumb for a substitution.

How much will 305/55R20 throw off my speedometer?

Every reading scales by 1.01847. An indicated 60 mph becomes 61.1 mph, and every 100 miles on the odometer is really 101.8 miles.

How much does 305/55R20 change ground clearance over 265/70R18?

The axle centerline moves by 0.30 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 305/55R20 fit where 265/70R18 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.

265/70R18 vs 305/55R20

305/55R20 is +0.60″ in overall diameter and +40 mm in section width against 265/70R18 — a diameter change of +1.85%, which stays inside the ±3% substitution rule. An indicated 60 mph would really be 61.1 mph.

Within 3%

Every dimension

Difference between 265/70R18 and 305/55R20
Dimension	265/70R18 → 305/55R20	Change
Overall diameter	32.61″ → 33.21″	+0.60″ (+1.85%)
Section width	265 mm → 305 mm	+40 mm (+15.09%)
Sidewall height	185.5 mm → 167.8 mm	−17.8 mm
Rolling circumference	102.44″ → 104.33″	+1.89″
Revolutions per mile	618.5 → 607.3	−11.2
Axle height / clearance	16.30″ → 16.60″	+0.30″

265/70R18265 mm

305/55R20305 mm

265/70R18 32.61″305/55R20 33.21″

Which is taller, 265/70R18 or 305/55R20?

305/55R20 is the taller of the two, at 33.21″ against 32.61″. The gap is 0.60 inches, or 15.3 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. 265/70R18 is 265 mm wide with a 185.5 mm sidewall on a 18″ wheel; 305/55R20 is 305 mm wide with a 167.8 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	25.5 mph	+0.5 mph
40 mph	40.7 mph	+0.7 mph
55 mph	56.0 mph	+1.0 mph
60 mph	61.1 mph	+1.1 mph
70 mph	71.3 mph	+1.3 mph
80 mph	81.5 mph	+1.5 mph

Fitting 305/55R20 in place of 265/70R18 multiplies every speed and distance reading by 1.01847. Over 10,000 indicated miles the odometer is out by +185 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 265/70R18 would need 3.80 to keep the same engine speed on 305/55R20. Work it out for your own axle ratio.

Full specification of each size

265/70R18

Derived dimensions for 265/70R18
Dimension	Metric	Imperial
Section width	265.0 mm	10.43″
Sidewall height	185.5 mm	7.30″
Wheel diameter	457.2 mm	18.0″
Overall diameter	828.2 mm	32.61″
Rolling circumference	2,601.9 mm	102.44″
Revolutions per mile	384.3 / km	618.5 / mile
Measuring rim (estimate)	203 mm	8″

305/55R20

Derived dimensions for 305/55R20
Dimension	Metric	Imperial
Section width	305.0 mm	12.01″
Sidewall height	167.8 mm	6.60″
Wheel diameter	508.0 mm	20.0″
Overall diameter	843.5 mm	33.21″
Rolling circumference	2,649.9 mm	104.33″
Revolutions per mile	377.4 / km	607.3 / mile
Measuring rim (estimate)	241 mm	9.5″

Rim widths differ too: 265/70R18 is designed for 7.5″–9″ and 305/55R20 for 9″–10.5″. 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 +1.85% 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 +40 mm is a separate question and is the one that decides whether it rubs.

Neither figure says anything about physical fitment. Whether 305/55R20 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.