Tire Size Box

305/40R19 vs 305/45R19

305/45R19 compared with 305/40R19

+4.2% diameter, 0.0" width

305/45R19 is 29.81" tall vs 28.61". More than the 3% rule of thumb, so check carefully before switching.

28.6"19" rim4.8"12.0" wide
305/40R19 (solid) compared with 305/45R19 (dashed outline), drawn to scale

With 305/45R19 tires, when your speedometer says 60 mph you are really going

62.5 mph

305/45R19 is +1.20 inches (+4.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/40R19305/45R19Difference%
Overall diameter28.61"29.81"+1.20"+4.2%
Section width12.01"12.01"0.00"0.0%
Sidewall height4.80"5.40"+0.60"+12.5%
Circumference89.87"93.64"+3.77"+4.2%
Revolutions per mile705.0676.6−28.4−4.0%

Speedometer difference

If your vehicle came with 305/40R19 and you fit 305/45R19, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.3 mph+1.3 mph
40 mph41.7 mph+1.7 mph
50 mph52.1 mph+2.1 mph
60 mph62.5 mph+2.5 mph
65 mph67.7 mph+2.7 mph
70 mph72.9 mph+2.9 mph
75 mph78.1 mph+3.1 mph
80 mph83.4 mph+3.4 mph

Each size on its own

Questions

Which is bigger, 305/40R19 or 305/45R19?
305/45R19 is taller by 1.20 inches (29.81" vs 28.61"). 305/40R19 is the same width.
Can I replace 305/40R19 with 305/45R19?
It is not recommended without checking carefully. The diameter difference is +4.2%, 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 305/45R19 change my speedometer?
When your speedometer reads 60 mph with 305/45R19 fitted (calibrated for 305/40R19), your real speed is 62.5 mph. Your odometer will under-count miles by about 4.2%.
Will 305/45R19 lift my vehicle?
Ride height changes by half the diameter difference: +0.60 inches of ground clearance at the axle.

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