Tire Size Box

305/30R19 vs 305/40R19

305/40R19 compared with 305/30R19

+9.2% diameter, 0.0" width

305/40R19 is 28.61" tall vs 26.20". More than the 3% rule of thumb, so check carefully before switching.

26.2"19" rim3.6"12.0" wide
305/30R19 (solid) compared with 305/40R19 (dashed outline), drawn to scale

With 305/40R19 tires, when your speedometer says 60 mph you are really going

65.5 mph

305/40R19 is +2.40 inches (+9.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/30R19305/40R19Difference%
Overall diameter26.20"28.61"+2.40"+9.2%
Section width12.01"12.01"0.00"0.0%
Sidewall height3.60"4.80"+1.20"+33.3%
Circumference82.32"89.87"+7.54"+9.2%
Revolutions per mile769.6705.0−64.6−8.4%

Speedometer difference

If your vehicle came with 305/30R19 and you fit 305/40R19, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.8 mph+1.8 mph
30 mph32.7 mph+2.7 mph
40 mph43.7 mph+3.7 mph
50 mph54.6 mph+4.6 mph
60 mph65.5 mph+5.5 mph
65 mph71.0 mph+6.0 mph
70 mph76.4 mph+6.4 mph
75 mph81.9 mph+6.9 mph
80 mph87.3 mph+7.3 mph

Each size on its own

Questions

Which is bigger, 305/30R19 or 305/40R19?
305/40R19 is taller by 2.40 inches (28.61" vs 26.20"). 305/30R19 is the same width.
Can I replace 305/30R19 with 305/40R19?
It is not recommended without checking carefully. The diameter difference is +9.2%, more than the 3% most shops and makers allow. It will throw off the speedometer and may affect ABS, stability control and clearance.
How much will 305/40R19 change my speedometer?
When your speedometer reads 60 mph with 305/40R19 fitted (calibrated for 305/30R19), your real speed is 65.5 mph. Your odometer will under-count miles by about 9.2%.
Will 305/40R19 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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