Tire Size Box

305/40R18 vs 275/40R19

275/40R19 compared with 305/40R18

+0.2% diameter, −1.2" width

275/40R19 is 27.66" tall vs 27.61". Within the 3% rule, so usually interchangeable.

27.6"18" rim4.8"12.0" wide
305/40R18 (solid) compared with 275/40R19 (dashed outline), drawn to scale

With 275/40R19 tires, when your speedometer says 60 mph you are really going

60.1 mph

275/40R19 is +0.06 inches (+0.2%) in overall diameter. That is within the 3% most shops and makers allow.

305/40R18275/40R19Difference%
Overall diameter27.61"27.66"+0.06"+0.2%
Section width12.01"10.83"−1.18"−9.8%
Sidewall height4.80"4.33"−0.47"−9.8%
Circumference86.73"86.90"+0.17"+0.2%
Revolutions per mile730.6729.1−1.5−0.2%

Speedometer difference

If your vehicle came with 305/40R18 and you fit 275/40R19, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.0 mph0.0 mph
30 mph30.1 mph+0.1 mph
40 mph40.1 mph+0.1 mph
50 mph50.1 mph+0.1 mph
60 mph60.1 mph+0.1 mph
65 mph65.1 mph+0.1 mph
70 mph70.1 mph+0.1 mph
75 mph75.1 mph+0.1 mph
80 mph80.2 mph+0.2 mph

Each size on its own

Questions

Which is bigger, 305/40R18 or 275/40R19?
275/40R19 is taller by 0.06 inches (27.66" vs 27.61"). 305/40R18 is wider by 1.18 inches.
Can I replace 305/40R18 with 275/40R19?
Usually, yes. The diameter difference is +0.2%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 275/40R19 change my speedometer?
When your speedometer reads 60 mph with 275/40R19 fitted (calibrated for 305/40R18), your real speed is 60.1 mph. Your odometer will under-count miles by about 0.2%.
Will 275/40R19 lift my vehicle?
Ride height changes by half the diameter difference: +0.03 inches of ground clearance at the axle.

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