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225/55R18 vs 345/25R21 tire size comparison

225/55R18345/25R21

345/25R21 is 0.1 in (0.17%) taller than 225/55R18 and 4.7 in wider. They fit different wheels: 225/55R18 an 18-inch wheel and 345/25R21 a 21-inch wheel.

225/55R18 and 345/25R21 side by side

+0.1 in / +0.17%225/55R18: 27.7 in tall, solid line345/25R21: 27.8 in tall, dashed line225/55R18Width 8.9 in (225 mm)Sidewall 4.9 in (124 mm)345/25R21Width 13.6 in (345 mm)Sidewall 3.4 in (86 mm)
225/55R18 is drawn with a solid line and 345/25R21 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 225/55R18 and 345/25R21, with the difference (345/25R21 minus 225/55R18)
Measurement225/55R18345/25R21DifferenceDifference %
Section width8.9 in (225 mm)13.6 in (345 mm)+4.7 in (+120 mm)+53.33%
Sidewall height4.9 in (124 mm)3.4 in (86 mm)-1.5 in (-38 mm)-30.30%
Wheel (rim) diameter18.0 in (457 mm)21.0 in (533 mm)+3.0 in (+76 mm)+16.67%
Overall diameter27.7 in (705 mm)27.8 in (706 mm)+0.1 in (+1 mm)+0.17%
Circumference87.2 in (2214 mm)87.3 in (2218 mm)+0.1 in (+4 mm)+0.17%
Revolutions per mile727726-1-0.17%

Differences are 345/25R21 minus 225/55R18; percentages are relative to 225/55R18. These are nominal design dimensions. Real tires vary by brand, tread depth, wheel width, load and pressure.

The overall diameter differs by 0.17%, 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 225/55R18, the actual speed on 345/25R21 is about 60.1 mph. When it reads 60 mph on a vehicle built for 345/25R21, the actual speed on 225/55R18 is about 59.9 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, 225/55R18 against 345/25R21
Speedometer readsActual on 345/25R21 (built for 225/55R18)Actual on 225/55R18 (built for 345/25R21)
20 mph20.0 mph20.0 mph
30 mph30.1 mph29.9 mph
40 mph40.1 mph39.9 mph
50 mph50.1 mph49.9 mph
60 mph60.1 mph59.9 mph
70 mph70.1 mph69.9 mph
80 mph80.1 mph79.9 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 225/55R18 or 345/25R21 tires

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Other comparisons with 225/55R18

Side-by-side dimensions and speedometer difference for sizes close to 225/55R18 in overall diameter.

Other comparisons with 345/25R21

Side-by-side dimensions and speedometer difference for sizes close to 345/25R21 in overall diameter.

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