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275/40R21 vs 275/45R20 tire size comparison

275/40R21275/45R20

275/45R20 is 3 mm (0.28%) taller than 275/40R21, with the same section width. They fit different wheels: 275/40R21 a 21-inch wheel and 275/45R20 a 20-inch wheel.

275/40R21 and 275/45R20 side by side

0.0 in / +0.28%275/40R21: 29.7 in tall, solid line275/45R20: 29.7 in tall, dashed line275/40R21Width 10.8 in (275 mm)Sidewall 4.3 in (110 mm)275/45R20Width 10.8 in (275 mm)Sidewall 4.9 in (124 mm)
275/40R21 is drawn with a solid line and 275/45R20 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 275/40R21 and 275/45R20, with the difference (275/45R20 minus 275/40R21)
Measurement275/40R21275/45R20DifferenceDifference %
Section width10.8 in (275 mm)10.8 in (275 mm)0.0 in (0 mm)0.00%
Sidewall height4.3 in (110 mm)4.9 in (124 mm)+0.6 in (+14 mm)+12.50%
Wheel (rim) diameter21.0 in (533 mm)20.0 in (508 mm)-1.0 in (-25 mm)-4.76%
Overall diameter29.7 in (753 mm)29.7 in (756 mm)0.0 in (+3 mm)+0.28%
Circumference93.2 in (2367 mm)93.4 in (2373 mm)+0.2 in (+6 mm)+0.28%
Revolutions per mile680678-2-0.28%

Differences are 275/45R20 minus 275/40R21; percentages are relative to 275/40R21. These are nominal design dimensions. Real tires vary by brand, tread depth, wheel width, load and pressure.

The overall diameter differs by 0.28%, 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 275/40R21, the actual speed on 275/45R20 is about 60.2 mph. When it reads 60 mph on a vehicle built for 275/45R20, the actual speed on 275/40R21 is about 59.8 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, 275/40R21 against 275/45R20
Speedometer readsActual on 275/45R20 (built for 275/40R21)Actual on 275/40R21 (built for 275/45R20)
20 mph20.1 mph19.9 mph
30 mph30.1 mph29.9 mph
40 mph40.1 mph39.9 mph
50 mph50.1 mph49.9 mph
60 mph60.2 mph59.8 mph
70 mph70.2 mph69.8 mph
80 mph80.2 mph79.8 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 275/40R21 or 275/45R20 tires

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Other comparisons with 275/40R21

Side-by-side dimensions and speedometer difference for sizes close to 275/40R21 in overall diameter.

Other comparisons with 275/45R20

Side-by-side dimensions and speedometer difference for sizes close to 275/45R20 in overall diameter.

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