Tire Size Box

305/60R18 vs 305/70R18

305/70R18 compared with 305/60R18

+7.4% diameter, 0.0" width

305/70R18 is 34.81" tall vs 32.41". More than the 3% rule of thumb, so check carefully before switching.

32.4"18" rim7.2"12.0" wide
305/60R18 (solid) compared with 305/70R18 (dashed outline), drawn to scale

With 305/70R18 tires, when your speedometer says 60 mph you are really going

64.4 mph

305/70R18 is +2.40 inches (+7.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/60R18305/70R18Difference%
Overall diameter32.41"34.81"+2.40"+7.4%
Section width12.01"12.01"0.00"0.0%
Sidewall height7.20"8.41"+1.20"+16.7%
Circumference101.82"109.36"+7.54"+7.4%
Revolutions per mile622.3579.4−42.9−6.9%

Speedometer difference

If your vehicle came with 305/60R18 and you fit 305/70R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.5 mph+1.5 mph
30 mph32.2 mph+2.2 mph
40 mph43.0 mph+3.0 mph
50 mph53.7 mph+3.7 mph
60 mph64.4 mph+4.4 mph
65 mph69.8 mph+4.8 mph
70 mph75.2 mph+5.2 mph
75 mph80.6 mph+5.6 mph
80 mph85.9 mph+5.9 mph

Each size on its own

Questions

Which is bigger, 305/60R18 or 305/70R18?
305/70R18 is taller by 2.40 inches (34.81" vs 32.41"). 305/60R18 is the same width.
Can I replace 305/60R18 with 305/70R18?
It is not recommended without checking carefully. The diameter difference is +7.4%, 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 305/70R18 change my speedometer?
When your speedometer reads 60 mph with 305/70R18 fitted (calibrated for 305/60R18), your real speed is 64.4 mph. Your odometer will under-count miles by about 7.4%.
Will 305/70R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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