Is 245/35R20 bigger than 255/35R19?

245/35R20 is 0.72 inches taller and 10 mm narrower. In overall diameter that is +2.78%, which stays inside the ±3% rule of thumb for a substitution.

How much will 245/35R20 throw off my speedometer?

Every reading scales by 1.02783. An indicated 60 mph becomes 61.7 mph, and every 100 miles on the odometer is really 102.8 miles.

How much does 245/35R20 change ground clearance over 255/35R19?

The axle centerline moves by 0.36 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 245/35R20 fit where 255/35R19 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.

255/35R19 vs 245/35R20

245/35R20 is +0.72″ in overall diameter and −10 mm in section width against 255/35R19 — a diameter change of +2.78%, which stays inside the ±3% substitution rule. An indicated 60 mph would really be 61.7 mph.

Within 3%

Every dimension

Difference between 255/35R19 and 245/35R20
Dimension	255/35R19 → 245/35R20	Change
Overall diameter	26.03″ → 26.75″	+0.72″ (+2.78%)
Section width	255 mm → 245 mm	−10 mm (−3.92%)
Sidewall height	89.2 mm → 85.8 mm	−3.5 mm
Rolling circumference	81.77″ → 84.04″	+2.28″
Revolutions per mile	774.9 → 753.9	−21.0
Axle height / clearance	13.01″ → 13.38″	+0.36″

255/35R19255 mm

245/35R20245 mm

255/35R19 26.03″245/35R20 26.75″

Which is taller, 255/35R19 or 245/35R20?

245/35R20 is the taller of the two, at 26.75″ against 26.03″. The gap is 0.72 inches, or 18.4 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. 255/35R19 is 255 mm wide with a 89.2 mm sidewall on a 19″ wheel; 245/35R20 is 245 mm wide with a 85.8 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	25.7 mph	+0.7 mph
40 mph	41.1 mph	+1.1 mph
55 mph	56.5 mph	+1.5 mph
60 mph	61.7 mph	+1.7 mph
70 mph	71.9 mph	+1.9 mph
80 mph	82.2 mph	+2.2 mph

Fitting 245/35R20 in place of 255/35R19 multiplies every speed and distance reading by 1.02783. Over 10,000 indicated miles the odometer is out by +278 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 255/35R19 would need 3.83 to keep the same engine speed on 245/35R20. Work it out for your own axle ratio.

Full specification of each size

255/35R19

Derived dimensions for 255/35R19
Dimension	Metric	Imperial
Section width	255.0 mm	10.04″
Sidewall height	89.2 mm	3.51″
Wheel diameter	482.6 mm	19.0″
Overall diameter	661.1 mm	26.03″
Rolling circumference	2,076.9 mm	81.77″
Revolutions per mile	481.5 / km	774.9 / mile
Measuring rim (estimate)	229 mm	9″

245/35R20

Derived dimensions for 245/35R20
Dimension	Metric	Imperial
Section width	245.0 mm	9.65″
Sidewall height	85.8 mm	3.38″
Wheel diameter	508.0 mm	20.0″
Overall diameter	679.5 mm	26.75″
Rolling circumference	2,134.7 mm	84.04″
Revolutions per mile	468.4 / km	753.9 / mile
Measuring rim (estimate)	216 mm	8.5″

Rim widths differ too: 255/35R19 is designed for 8.5″–10″ and 245/35R20 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?

Inside the ±3% rule

The diameter difference of +2.78% 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 −10 mm is a separate question and is the one that decides whether it rubs.

Neither figure says anything about physical fitment. Whether 245/35R20 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.