Is 255/35R19 bigger than 275/40R18?

255/35R19 is 0.63 inches shorter and 20 mm narrower. In overall diameter that is −2.38%, which stays inside the ±3% rule of thumb for a substitution.

How much will 255/35R19 throw off my speedometer?

Every reading scales by 0.97623. An indicated 60 mph becomes 58.6 mph, and every 100 miles on the odometer is really 97.6 miles.

How much does 255/35R19 change ground clearance over 275/40R18?

The axle centerline moves by 0.32 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 255/35R19 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 255/35R19

255/35R19 is −0.63″ in overall diameter and −20 mm in section width against 275/40R18 — a diameter change of −2.38%, which stays inside the ±3% substitution rule. An indicated 60 mph would really be 58.6 mph.

Within 3%

Every dimension

Difference between 275/40R18 and 255/35R19
Dimension	275/40R18 → 255/35R19	Change
Overall diameter	26.66″ → 26.03″	−0.63″ (−2.38%)
Section width	275 mm → 255 mm	−20 mm (−7.27%)
Sidewall height	110.0 mm → 89.2 mm	−20.8 mm
Rolling circumference	83.76″ → 81.77″	−1.99″
Revolutions per mile	756.5 → 774.9	+18.4
Axle height / clearance	13.33″ → 13.01″	−0.32″

275/40R18275 mm

255/35R19255 mm

275/40R18 26.66″255/35R19 26.03″

Which is taller, 275/40R18 or 255/35R19?

275/40R18 is the taller of the two, at 26.66″ against 26.03″. The gap is 0.63 inches, or 16.1 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; 255/35R19 is 255 mm wide with a 89.2 mm sidewall on a 19″ wheel.

What the speedometer will read

Indicated versus actual speed after the size change
Speedometer reads	Actual speed	Error
25 mph	24.4 mph	−0.6 mph
40 mph	39.0 mph	−1.0 mph
55 mph	53.7 mph	−1.3 mph
60 mph	58.6 mph	−1.4 mph
70 mph	68.3 mph	−1.7 mph
80 mph	78.1 mph	−1.9 mph

Fitting 255/35R19 in place of 275/40R18 multiplies every speed and distance reading by 0.97623. Over 10,000 indicated miles the odometer is out by −238 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 275/40R18 would need 3.64 to keep the same engine speed on 255/35R19. 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″

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″

Rim widths differ too: 275/40R18 is designed for 9″–10.5″ and 255/35R19 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 −2.38% 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 255/35R19 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.