Tire Size Box

235/75R17 vs 235/70R18

235/70R18 compared with 235/75R17

+0.2% diameter, 0.0" width

235/70R18 is 30.95" tall vs 30.88". Within the 3% rule, so usually interchangeable.

30.9"17" rim6.9"9.3" wide
235/75R17 (solid) compared with 235/70R18 (dashed outline), drawn to scale

With 235/70R18 tires, when your speedometer says 60 mph you are really going

60.1 mph

235/70R18 is +0.07 inches (+0.2%) in overall diameter. That is within the 3% most shops and makers allow.

235/75R17235/70R18Difference%
Overall diameter30.88"30.95"+0.07"+0.2%
Section width9.25"9.25"0.00"0.0%
Sidewall height6.94"6.48"−0.46"−6.7%
Circumference97.01"97.24"+0.24"+0.2%
Revolutions per mile653.2651.6−1.6−0.2%

Speedometer difference

If your vehicle came with 235/75R17 and you fit 235/70R18, 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 mph65.2 mph+0.2 mph
70 mph70.2 mph+0.2 mph
75 mph75.2 mph+0.2 mph
80 mph80.2 mph+0.2 mph

Each size on its own

Questions

Which is bigger, 235/75R17 or 235/70R18?
235/70R18 is taller by 0.07 inches (30.95" vs 30.88"). 235/75R17 is the same width.
Can I replace 235/75R17 with 235/70R18?
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 235/70R18 change my speedometer?
When your speedometer reads 60 mph with 235/70R18 fitted (calibrated for 235/75R17), your real speed is 60.1 mph. Your odometer will under-count miles by about 0.2%.
Will 235/70R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.04 inches of ground clearance at the axle.

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