Is 275/65R18 bigger than 245/75R16?

275/65R18 is 1.61 inches taller and 30 mm wider. In overall diameter that is +5.27%, which exceeds the ±3% rule of thumb for a substitution.

How much will 275/65R18 throw off my speedometer?

Every reading scales by 1.05272. An indicated 60 mph becomes 63.2 mph, and every 100 miles on the odometer is really 105.3 miles.

How much does 275/65R18 change ground clearance over 245/75R16?

The axle centerline moves by 0.80 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 275/65R18 fit where 245/75R16 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.

245/75R16 vs 275/65R18

275/65R18 is +1.61″ in overall diameter and +30 mm in section width against 245/75R16 — a diameter change of +5.27%, which breaks the ±3% substitution rule. An indicated 60 mph would really be 63.2 mph.

Outside 3%

Every dimension

Difference between 245/75R16 and 275/65R18
Dimension	245/75R16 → 275/65R18	Change
Overall diameter	30.47″ → 32.07″	+1.61″ (+5.27%)
Section width	245 mm → 275 mm	+30 mm (+12.24%)
Sidewall height	183.8 mm → 178.8 mm	−5.0 mm
Rolling circumference	95.72″ → 100.77″	+5.05″
Revolutions per mile	661.9 → 628.8	−33.1
Axle height / clearance	15.23″ → 16.04″	+0.80″

245/75R16245 mm

275/65R18275 mm

245/75R16 30.47″275/65R18 32.07″

Which is taller, 245/75R16 or 275/65R18?

275/65R18 is the taller of the two, at 32.07″ against 30.47″. The gap is 1.61 inches, or 40.8 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. 245/75R16 is 245 mm wide with a 183.8 mm sidewall on a 16″ wheel; 275/65R18 is 275 mm wide with a 178.8 mm sidewall on a 18″ wheel.

What the speedometer will read

Indicated versus actual speed after the size change
Speedometer reads	Actual speed	Error
25 mph	26.3 mph	+1.3 mph
40 mph	42.1 mph	+2.1 mph
55 mph	57.9 mph	+2.9 mph
60 mph	63.2 mph	+3.2 mph
70 mph	73.7 mph	+3.7 mph
80 mph	84.2 mph	+4.2 mph

Fitting 275/65R18 in place of 245/75R16 multiplies every speed and distance reading by 1.05272. Over 10,000 indicated miles the odometer is out by +527 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 245/75R16 would need 3.93 to keep the same engine speed on 275/65R18. Work it out for your own axle ratio.

Full specification of each size

245/75R16

Derived dimensions for 245/75R16
Dimension	Metric	Imperial
Section width	245.0 mm	9.65″
Sidewall height	183.8 mm	7.23″
Wheel diameter	406.4 mm	16.0″
Overall diameter	773.9 mm	30.47″
Rolling circumference	2,431.3 mm	95.72″
Revolutions per mile	411.3 / km	661.9 / mile
Measuring rim (estimate)	178 mm	7″

275/65R18

Derived dimensions for 275/65R18
Dimension	Metric	Imperial
Section width	275.0 mm	10.83″
Sidewall height	178.8 mm	7.04″
Wheel diameter	457.2 mm	18.0″
Overall diameter	814.7 mm	32.07″
Rolling circumference	2,559.5 mm	100.77″
Revolutions per mile	390.7 / km	628.8 / mile
Measuring rim (estimate)	216 mm	8.5″

Rim widths differ too: 245/75R16 is designed for 6.5″–8″ and 275/65R18 for 8″–9.5″. 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 +5.27% exceeds the ±3% rule of thumb. Expect a speedometer error of about 3.2 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 275/65R18 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.