Is 275/45R20 bigger than 255/55R19?

275/45R20 is 0.30 inches shorter and 20 mm wider. In overall diameter that is −1.00%, which stays inside the ±3% rule of thumb for a substitution.

How much will 275/45R20 throw off my speedometer?

Every reading scales by 0.99004. An indicated 60 mph becomes 59.4 mph, and every 100 miles on the odometer is really 99.0 miles.

How much does 275/45R20 change ground clearance over 255/55R19?

The axle centerline moves by 0.15 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 275/45R20 fit where 255/55R19 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.

255/55R19 vs 275/45R20

275/45R20 is −0.30″ in overall diameter and +20 mm in section width against 255/55R19 — a diameter change of −1.00%, which stays inside the ±3% substitution rule. An indicated 60 mph would really be 59.4 mph.

Within 3%

Every dimension

Difference between 255/55R19 and 275/45R20
Dimension	255/55R19 → 275/45R20	Change
Overall diameter	30.04″ → 29.74″	−0.30″ (−1.00%)
Section width	255 mm → 275 mm	+20 mm (+7.84%)
Sidewall height	140.2 mm → 123.8 mm	−16.5 mm
Rolling circumference	94.38″ → 93.44″	−0.94″
Revolutions per mile	671.3 → 678.1	+6.8
Axle height / clearance	15.02″ → 14.87″	−0.15″

255/55R19255 mm

275/45R20275 mm

255/55R19 30.04″275/45R20 29.74″

Which is taller, 255/55R19 or 275/45R20?

255/55R19 is the taller of the two, at 30.04″ against 29.74″. The gap is 0.30 inches, or 7.6 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. 255/55R19 is 255 mm wide with a 140.2 mm sidewall on a 19″ wheel; 275/45R20 is 275 mm wide with a 123.8 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	24.8 mph	−0.2 mph
40 mph	39.6 mph	−0.4 mph
55 mph	54.5 mph	−0.5 mph
60 mph	59.4 mph	−0.6 mph
70 mph	69.3 mph	−0.7 mph
80 mph	79.2 mph	−0.8 mph

Fitting 275/45R20 in place of 255/55R19 multiplies every speed and distance reading by 0.99004. Over 10,000 indicated miles the odometer is out by −100 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 255/55R19 would need 3.69 to keep the same engine speed on 275/45R20. Work it out for your own axle ratio.

Full specification of each size

255/55R19

Derived dimensions for 255/55R19
Dimension	Metric	Imperial
Section width	255.0 mm	10.04″
Sidewall height	140.2 mm	5.52″
Wheel diameter	482.6 mm	19.0″
Overall diameter	763.1 mm	30.04″
Rolling circumference	2,397.3 mm	94.38″
Revolutions per mile	417.1 / km	671.3 / mile
Measuring rim (estimate)	203 mm	8″

275/45R20

Derived dimensions for 275/45R20
Dimension	Metric	Imperial
Section width	275.0 mm	10.83″
Sidewall height	123.8 mm	4.87″
Wheel diameter	508.0 mm	20.0″
Overall diameter	755.5 mm	29.74″
Rolling circumference	2,373.5 mm	93.44″
Revolutions per mile	421.3 / km	678.1 / mile
Measuring rim (estimate)	229 mm	9″

Rim widths differ too: 255/55R19 is designed for 7.5″–9″ and 275/45R20 for 8.5″–10″. Their measuring rims differ, so the same wheel will not be ideal for both.

Can you swap one for the other?

Inside the ±3% rule

The diameter difference of −1.00% 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 275/45R20 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.