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

255/35R19 is 0.00 inches shorter and 0 mm wider. In overall diameter that is −0.02%, which stays inside the ±3% rule of thumb for a substitution.

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

Every reading scales by 0.99985. An indicated 60 mph becomes 60.0 mph, and every 100 miles on the odometer is really 100.0 miles.

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

The axle centerline moves by 0.00 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 255/35R19 fit where 255/40R18 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.

255/40R18 vs 255/35R19

255/35R19 is −0.00″ in overall diameter and 0 mm in section width against 255/40R18 — a diameter change of −0.02%, which stays inside the ±3% substitution rule. An indicated 60 mph would really be 60.0 mph.

Within 3%

Every dimension

Difference between 255/40R18 and 255/35R19
Dimension	255/40R18 → 255/35R19	Change
Overall diameter	26.03″ → 26.03″	−0.00″ (−0.02%)
Section width	255 mm → 255 mm	0 mm (0.00%)
Sidewall height	102.0 mm → 89.2 mm	−12.8 mm
Rolling circumference	81.78″ → 81.77″	−0.01″
Revolutions per mile	774.8 → 774.9	+0.1
Axle height / clearance	13.02″ → 13.01″	−0.00″

255/40R18255 mm

255/35R19255 mm

255/40R18 26.03″255/35R19 26.03″

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

255/40R18 is the taller of the two, at 26.03″ against 26.03″. The gap is 0.00 inches, or 0.1 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. 255/40R18 is 255 mm wide with a 102.0 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	25.0 mph	−0.0 mph
40 mph	40.0 mph	−0.0 mph
55 mph	55.0 mph	−0.0 mph
60 mph	60.0 mph	−0.0 mph
70 mph	70.0 mph	−0.0 mph
80 mph	80.0 mph	−0.0 mph

Fitting 255/35R19 in place of 255/40R18 multiplies every speed and distance reading by 0.99985. Over 10,000 indicated miles the odometer is out by −2 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 255/40R18 would need 3.73 to keep the same engine speed on 255/35R19. Work it out for your own axle ratio.

Full specification of each size

255/40R18

Derived dimensions for 255/40R18
Dimension	Metric	Imperial
Section width	255.0 mm	10.04″
Sidewall height	102.0 mm	4.02″
Wheel diameter	457.2 mm	18.0″
Overall diameter	661.2 mm	26.03″
Rolling circumference	2,077.2 mm	81.78″
Revolutions per mile	481.4 / km	774.8 / mile
Measuring rim (estimate)	229 mm	9″

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: 255/40R18 is designed for 8.5″–10″ and 255/35R19 for 8.5″–10″. They share a measuring rim, so one wheel can usually carry either.

Can you swap one for the other?

Inside the ±3% rule

The diameter difference of −0.02% 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 0 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.