Is 245/50R20 bigger than 265/60R18?

245/50R20 is 0.87 inches shorter and 20 mm narrower. In overall diameter that is −2.86%, which stays inside the ±3% rule of thumb for a substitution.

How much will 245/50R20 throw off my speedometer?

Every reading scales by 0.97136. An indicated 60 mph becomes 58.3 mph, and every 100 miles on the odometer is really 97.1 miles.

How much does 245/50R20 change ground clearance over 265/60R18?

The axle centerline moves by 0.44 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 245/50R20 fit where 265/60R18 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.

265/60R18 vs 245/50R20

245/50R20 is −0.87″ in overall diameter and −20 mm in section width against 265/60R18 — a diameter change of −2.86%, which stays inside the ±3% substitution rule. An indicated 60 mph would really be 58.3 mph.

Within 3%

Every dimension

Difference between 265/60R18 and 245/50R20
Dimension	265/60R18 → 245/50R20	Change
Overall diameter	30.52″ → 29.65″	−0.87″ (−2.86%)
Section width	265 mm → 245 mm	−20 mm (−7.55%)
Sidewall height	159.0 mm → 122.5 mm	−36.5 mm
Rolling circumference	95.88″ → 93.13″	−2.75″
Revolutions per mile	660.8 → 680.3	+19.5
Axle height / clearance	15.26″ → 14.82″	−0.44″

265/60R18265 mm

245/50R20245 mm

265/60R18 30.52″245/50R20 29.65″

Which is taller, 265/60R18 or 245/50R20?

265/60R18 is the taller of the two, at 30.52″ against 29.65″. The gap is 0.87 inches, or 22.2 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. 265/60R18 is 265 mm wide with a 159.0 mm sidewall on a 18″ wheel; 245/50R20 is 245 mm wide with a 122.5 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	24.3 mph	−0.7 mph
40 mph	38.9 mph	−1.1 mph
55 mph	53.4 mph	−1.6 mph
60 mph	58.3 mph	−1.7 mph
70 mph	68.0 mph	−2.0 mph
80 mph	77.7 mph	−2.3 mph

Fitting 245/50R20 in place of 265/60R18 multiplies every speed and distance reading by 0.97136. Over 10,000 indicated miles the odometer is out by −286 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 265/60R18 would need 3.62 to keep the same engine speed on 245/50R20. Work it out for your own axle ratio.

Full specification of each size

265/60R18

Derived dimensions for 265/60R18
Dimension	Metric	Imperial
Section width	265.0 mm	10.43″
Sidewall height	159.0 mm	6.26″
Wheel diameter	457.2 mm	18.0″
Overall diameter	775.2 mm	30.52″
Rolling circumference	2,435.4 mm	95.88″
Revolutions per mile	410.6 / km	660.8 / mile
Measuring rim (estimate)	203 mm	8″

245/50R20

Derived dimensions for 245/50R20
Dimension	Metric	Imperial
Section width	245.0 mm	9.65″
Sidewall height	122.5 mm	4.82″
Wheel diameter	508.0 mm	20.0″
Overall diameter	753.0 mm	29.65″
Rolling circumference	2,365.6 mm	93.13″
Revolutions per mile	422.7 / km	680.3 / mile
Measuring rim (estimate)	203 mm	8″

Rim widths differ too: 265/60R18 is designed for 7.5″–9″ and 245/50R20 for 7.5″–9″. They share a measuring rim, so one wheel can usually carry either.

Can you swap one for the other?

Inside the ±3% rule

The diameter difference of −2.86% 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 −20 mm is a separate question and is the one that decides whether it rubs.

Neither figure says anything about physical fitment. Whether 245/50R20 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.