Is 325/60R20 bigger than 265/65R18?

325/60R20 is 3.79 inches taller and 60 mm wider. In overall diameter that is +12.01%, which exceeds the ±3% rule of thumb for a substitution.

How much will 325/60R20 throw off my speedometer?

Every reading scales by 1.12012. An indicated 60 mph becomes 67.2 mph, and every 100 miles on the odometer is really 112.0 miles.

How much does 325/60R20 change ground clearance over 265/65R18?

The axle centerline moves by 1.90 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 325/60R20 fit where 265/65R18 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.

265/65R18 vs 325/60R20

325/60R20 is +3.79″ in overall diameter and +60 mm in section width against 265/65R18 — a diameter change of +12.01%, which breaks the ±3% substitution rule. An indicated 60 mph would really be 67.2 mph.

Outside 3%

Every dimension

Difference between 265/65R18 and 325/60R20
Dimension	265/65R18 → 325/60R20	Change
Overall diameter	31.56″ → 35.35″	+3.79″ (+12.01%)
Section width	265 mm → 325 mm	+60 mm (+22.64%)
Sidewall height	172.3 mm → 195.0 mm	+22.7 mm
Rolling circumference	99.16″ → 111.07″	+11.91″
Revolutions per mile	639.0 → 570.5	−68.5
Axle height / clearance	15.78″ → 17.68″	+1.90″

265/65R18265 mm

325/60R20325 mm

265/65R18 31.56″325/60R20 35.35″

Which is taller, 265/65R18 or 325/60R20?

325/60R20 is the taller of the two, at 35.35″ against 31.56″. The gap is 3.79 inches, or 96.3 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. 265/65R18 is 265 mm wide with a 172.3 mm sidewall on a 18″ wheel; 325/60R20 is 325 mm wide with a 195.0 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	28.0 mph	+3.0 mph
40 mph	44.8 mph	+4.8 mph
55 mph	61.6 mph	+6.6 mph
60 mph	67.2 mph	+7.2 mph
70 mph	78.4 mph	+8.4 mph
80 mph	89.6 mph	+9.6 mph

Fitting 325/60R20 in place of 265/65R18 multiplies every speed and distance reading by 1.12012. Over 10,000 indicated miles the odometer is out by +1,201 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 265/65R18 would need 4.18 to keep the same engine speed on 325/60R20. Work it out for your own axle ratio.

Full specification of each size

265/65R18

Derived dimensions for 265/65R18
Dimension	Metric	Imperial
Section width	265.0 mm	10.43″
Sidewall height	172.3 mm	6.78″
Wheel diameter	457.2 mm	18.0″
Overall diameter	801.7 mm	31.56″
Rolling circumference	2,518.6 mm	99.16″
Revolutions per mile	397.0 / km	639.0 / mile
Measuring rim (estimate)	203 mm	8″

325/60R20

Derived dimensions for 325/60R20
Dimension	Metric	Imperial
Section width	325.0 mm	12.80″
Sidewall height	195.0 mm	7.68″
Wheel diameter	508.0 mm	20.0″
Overall diameter	898.0 mm	35.35″
Rolling circumference	2,821.2 mm	111.07″
Revolutions per mile	354.5 / km	570.5 / mile
Measuring rim (estimate)	254 mm	10″

Rim widths differ too: 265/65R18 is designed for 7.5″–9″ and 325/60R20 for 9.5″–11″. 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 +12.01% exceeds the ±3% rule of thumb. Expect a speedometer error of about 7.2 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 325/60R20 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.