Is 305/30R20 bigger than 245/40R18?

305/30R20 is 1.49 inches taller and 60 mm wider. In overall diameter that is +5.79%, which exceeds the ±3% rule of thumb for a substitution.

How much will 305/30R20 throw off my speedometer?

Every reading scales by 1.05787. An indicated 60 mph becomes 63.5 mph, and every 100 miles on the odometer is really 105.8 miles.

How much does 305/30R20 change ground clearance over 245/40R18?

The axle centerline moves by 0.74 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 305/30R20 fit where 245/40R18 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.

245/40R18 vs 305/30R20

305/30R20 is +1.49″ in overall diameter and +60 mm in section width against 245/40R18 — a diameter change of +5.79%, which breaks the ±3% substitution rule. An indicated 60 mph would really be 63.5 mph.

Outside 3%

Every dimension

Difference between 245/40R18 and 305/30R20
Dimension	245/40R18 → 305/30R20	Change
Overall diameter	25.72″ → 27.20″	+1.49″ (+5.79%)
Section width	245 mm → 305 mm	+60 mm (+24.49%)
Sidewall height	98.0 mm → 91.5 mm	−6.5 mm
Rolling circumference	80.79″ → 85.47″	+4.68″
Revolutions per mile	784.2 → 741.3	−42.9
Axle height / clearance	12.86″ → 13.60″	+0.74″

245/40R18245 mm

305/30R20305 mm

245/40R18 25.72″305/30R20 27.20″

Which is taller, 245/40R18 or 305/30R20?

305/30R20 is the taller of the two, at 27.20″ against 25.72″. The gap is 1.49 inches, or 37.8 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. 245/40R18 is 245 mm wide with a 98.0 mm sidewall on a 18″ wheel; 305/30R20 is 305 mm wide with a 91.5 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	26.4 mph	+1.4 mph
40 mph	42.3 mph	+2.3 mph
55 mph	58.2 mph	+3.2 mph
60 mph	63.5 mph	+3.5 mph
70 mph	74.1 mph	+4.1 mph
80 mph	84.6 mph	+4.6 mph

Fitting 305/30R20 in place of 245/40R18 multiplies every speed and distance reading by 1.05787. Over 10,000 indicated miles the odometer is out by +579 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 245/40R18 would need 3.95 to keep the same engine speed on 305/30R20. Work it out for your own axle ratio.

Full specification of each size

245/40R18

Derived dimensions for 245/40R18
Dimension	Metric	Imperial
Section width	245.0 mm	9.65″
Sidewall height	98.0 mm	3.86″
Wheel diameter	457.2 mm	18.0″
Overall diameter	653.2 mm	25.72″
Rolling circumference	2,052.1 mm	80.79″
Revolutions per mile	487.3 / km	784.2 / mile
Measuring rim (estimate)	216 mm	8.5″

305/30R20

Derived dimensions for 305/30R20
Dimension	Metric	Imperial
Section width	305.0 mm	12.01″
Sidewall height	91.5 mm	3.60″
Wheel diameter	508.0 mm	20.0″
Overall diameter	691.0 mm	27.20″
Rolling circumference	2,170.8 mm	85.47″
Revolutions per mile	460.7 / km	741.3 / mile
Measuring rim (estimate)	267 mm	10.5″

Rim widths differ too: 245/40R18 is designed for 8″–9.5″ and 305/30R20 for 10″–11.5″. 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 +5.79% exceeds the ±3% rule of thumb. Expect a speedometer error of about 3.5 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 305/30R20 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.