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215/55R18 vs 305/30R20 tire size comparison

215/55R18305/30R20

305/30R20 is 0.1 in (0.39%) shorter than 215/55R18 and 3.5 in wider. They fit different wheels: 215/55R18 an 18-inch wheel and 305/30R20 a 20-inch wheel.

215/55R18 and 305/30R20 side by side

-0.1 in / -0.39%215/55R18: 27.3 in tall, solid line305/30R20: 27.2 in tall, dashed line215/55R18Width 8.5 in (215 mm)Sidewall 4.7 in (118 mm)305/30R20Width 12.0 in (305 mm)Sidewall 3.6 in (92 mm)
215/55R18 is drawn with a solid line and 305/30R20 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 215/55R18 and 305/30R20, with the difference (305/30R20 minus 215/55R18)
Measurement215/55R18305/30R20DifferenceDifference %
Section width8.5 in (215 mm)12.0 in (305 mm)+3.5 in (+90 mm)+41.86%
Sidewall height4.7 in (118 mm)3.6 in (92 mm)-1.1 in (-26 mm)-22.62%
Wheel (rim) diameter18.0 in (457 mm)20.0 in (508 mm)+2.0 in (+51 mm)+11.11%
Overall diameter27.3 in (694 mm)27.2 in (691 mm)-0.1 in (-3 mm)-0.39%
Circumference85.8 in (2179 mm)85.5 in (2171 mm)-0.3 in (-8 mm)-0.39%
Revolutions per mile738741+3+0.39%

Differences are 305/30R20 minus 215/55R18; percentages are relative to 215/55R18. These are nominal design dimensions. Real tires vary by brand, tread depth, wheel width, load and pressure.

The overall diameter differs by 0.39%, 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 215/55R18, the actual speed on 305/30R20 is about 59.8 mph. When it reads 60 mph on a vehicle built for 305/30R20, the actual speed on 215/55R18 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, 215/55R18 against 305/30R20
Speedometer readsActual on 305/30R20 (built for 215/55R18)Actual on 215/55R18 (built for 305/30R20)
20 mph19.9 mph20.1 mph
30 mph29.9 mph30.1 mph
40 mph39.8 mph40.2 mph
50 mph49.8 mph50.2 mph
60 mph59.8 mph60.2 mph
70 mph69.7 mph70.3 mph
80 mph79.7 mph80.3 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

The calculator takes the same two sizes and shows the full comparison, or lets you swap either one for another size.

Open this comparison in the calculator

Shop for 215/55R18 or 305/30R20 tires

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Other comparisons with 215/55R18

Side-by-side dimensions and speedometer difference for sizes close to 215/55R18 in overall diameter.

Other comparisons with 305/30R20

Side-by-side dimensions and speedometer difference for sizes close to 305/30R20 in overall diameter.

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