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245/30R20 vs 245/40R18 tire size comparison

245/30R20245/40R18

245/40R18 is 0.1 in (0.27%) shorter than 245/30R20, with the same section width. They fit different wheels: 245/30R20 a 20-inch wheel and 245/40R18 an 18-inch wheel.

245/30R20 and 245/40R18 side by side

-0.1 in / -0.27%245/30R20: 25.8 in tall, solid line245/40R18: 25.7 in tall, dashed line245/30R20Width 9.6 in (245 mm)Sidewall 2.9 in (74 mm)245/40R18Width 9.6 in (245 mm)Sidewall 3.9 in (98 mm)
245/30R20 is drawn with a solid line and 245/40R18 with a dashed line. In each drawing both tires are to the same scale; the tread shoulder and bead curves are simplified. The table below has the same numbers.
Dimensions of 245/30R20 and 245/40R18, with the difference (245/40R18 minus 245/30R20)
Measurement245/30R20245/40R18DifferenceDifference %
Section width9.6 in (245 mm)9.6 in (245 mm)0.0 in (0 mm)0.00%
Sidewall height2.9 in (74 mm)3.9 in (98 mm)+1.0 in (+24 mm)+33.33%
Wheel (rim) diameter20.0 in (508 mm)18.0 in (457 mm)-2.0 in (-51 mm)-10.00%
Overall diameter25.8 in (655 mm)25.7 in (653 mm)-0.1 in (-2 mm)-0.27%
Circumference81.0 in (2058 mm)80.8 in (2052 mm)-0.2 in (-6 mm)-0.27%
Revolutions per mile782784+2+0.28%

Differences are 245/40R18 minus 245/30R20; percentages are relative to 245/30R20. These are nominal design dimensions. Real tires vary by brand, tread depth, wheel width, load and pressure.

The overall diameter differs by 0.27%, which is within the common 3% rule of thumb.

The rule of thumb keeps a replacement size within about 3% of the original overall diameter. It covers diameter only. It is not a fit, load or safety check, because width, wheel diameter, load capacity and clearance matter too. Confirm any size change with your vehicle's owner's manual or a tire professional.

Speedometer difference

When the speedometer reads 60 mph on a vehicle built for 245/30R20, the actual speed on 245/40R18 is about 59.8 mph. When it reads 60 mph on a vehicle built for 245/40R18, the actual speed on 245/30R20 is about 60.2 mph. These figures assume the speedometer is accurate on the original size and use nominal dimensions, so treat them as estimates.

Actual speed for a given speedometer reading, 245/30R20 against 245/40R18
Speedometer readsActual on 245/40R18 (built for 245/30R20)Actual on 245/30R20 (built for 245/40R18)
20 mph19.9 mph20.1 mph
30 mph29.9 mph30.1 mph
40 mph39.9 mph40.1 mph
50 mph49.9 mph50.1 mph
60 mph59.8 mph60.2 mph
70 mph69.8 mph70.2 mph
80 mph79.8 mph80.2 mph

A common rule of thumb is to keep a replacement size within about 3% of the original overall diameter. Check the tire placard on the driver's door jamb and your owner's manual for the sizes your vehicle was built for. Confirm any size change with your vehicle's owner's manual or a tire professional.

Which vehicles use each size?

These vehicles list the size among their verified sizes, by trim. Open one to see all of its sizes.

Check it in the calculator

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Shop for 245/30R20 or 245/40R18 tires

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Other comparisons with 245/30R20

Side-by-side dimensions and speedometer difference for sizes close to 245/30R20 in overall diameter.

Other comparisons with 245/40R18

Side-by-side dimensions and speedometer difference for sizes close to 245/40R18 in overall diameter.

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