Is 275/40R20 bigger than 275/40R18?

275/40R20 is 2.00 inches taller and 0 mm wider. In overall diameter that is +7.50%, which exceeds the ±3% rule of thumb for a substitution.

How much will 275/40R20 throw off my speedometer?

Every reading scales by 1.07501. An indicated 60 mph becomes 64.5 mph, and every 100 miles on the odometer is really 107.5 miles.

How much does 275/40R20 change ground clearance over 275/40R18?

The axle centerline moves by 1.00 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 275/40R20 fit where 275/40R18 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.

275/40R18 vs 275/40R20

275/40R20 is +2.00″ in overall diameter and 0 mm in section width against 275/40R18 — a diameter change of +7.50%, which breaks the ±3% substitution rule. An indicated 60 mph would really be 64.5 mph.

Outside 3%

Every dimension

Difference between 275/40R18 and 275/40R20
Dimension	275/40R18 → 275/40R20	Change
Overall diameter	26.66″ → 28.66″	+2.00″ (+7.50%)
Section width	275 mm → 275 mm	0 mm (0.00%)
Sidewall height	110.0 mm → 110.0 mm	−0.0 mm
Rolling circumference	83.76″ → 90.04″	+6.28″
Revolutions per mile	756.5 → 703.7	−52.8
Axle height / clearance	13.33″ → 14.33″	+1.00″

275/40R18275 mm

275/40R20275 mm

275/40R18 26.66″275/40R20 28.66″

Which is taller, 275/40R18 or 275/40R20?

275/40R20 is the taller of the two, at 28.66″ against 26.66″. The gap is 2.00 inches, or 50.8 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. 275/40R18 is 275 mm wide with a 110.0 mm sidewall on a 18″ wheel; 275/40R20 is 275 mm wide with a 110.0 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	26.9 mph	+1.9 mph
40 mph	43.0 mph	+3.0 mph
55 mph	59.1 mph	+4.1 mph
60 mph	64.5 mph	+4.5 mph
70 mph	75.3 mph	+5.3 mph
80 mph	86.0 mph	+6.0 mph

Fitting 275/40R20 in place of 275/40R18 multiplies every speed and distance reading by 1.07501. Over 10,000 indicated miles the odometer is out by +750 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 275/40R18 would need 4.01 to keep the same engine speed on 275/40R20. Work it out for your own axle ratio.

Full specification of each size

275/40R18

Derived dimensions for 275/40R18
Dimension	Metric	Imperial
Section width	275.0 mm	10.83″
Sidewall height	110.0 mm	4.33″
Wheel diameter	457.2 mm	18.0″
Overall diameter	677.2 mm	26.66″
Rolling circumference	2,127.5 mm	83.76″
Revolutions per mile	470.0 / km	756.5 / mile
Measuring rim (estimate)	241 mm	9.5″

275/40R20

Derived dimensions for 275/40R20
Dimension	Metric	Imperial
Section width	275.0 mm	10.83″
Sidewall height	110.0 mm	4.33″
Wheel diameter	508.0 mm	20.0″
Overall diameter	728.0 mm	28.66″
Rolling circumference	2,287.1 mm	90.04″
Revolutions per mile	437.2 / km	703.7 / mile
Measuring rim (estimate)	241 mm	9.5″

Rim widths differ too: 275/40R18 is designed for 9″–10.5″ and 275/40R20 for 9″–10.5″. They share a measuring rim, so one wheel can usually carry either.

Can you swap one for the other?

Outside the ±3% rule

The diameter difference of +7.50% exceeds the ±3% rule of thumb. Expect a speedometer error of about 4.5 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 275/40R20 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.