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235/65R17 vs 255/40R21 tire size comparison

235/65R17255/40R21

255/40R21 is 0.00 in (0.01%) taller than 235/65R17 and 0.7 in wider. They fit different wheels: 235/65R17 a 17-inch wheel and 255/40R21 a 21-inch wheel.

235/65R17 and 255/40R21 side by side

0.0 in / +0.01%235/65R17: 29.0 in tall, solid line255/40R21: 29.0 in tall, dashed line235/65R17Width 9.3 in (235 mm)Sidewall 6.0 in (153 mm)255/40R21Width 10.0 in (255 mm)Sidewall 4.0 in (102 mm)
235/65R17 is drawn with a solid line and 255/40R21 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 235/65R17 and 255/40R21, with the difference (255/40R21 minus 235/65R17)
Measurement235/65R17255/40R21DifferenceDifference %
Section width9.3 in (235 mm)10.0 in (255 mm)+0.7 in (+20 mm)+8.51%
Sidewall height6.0 in (153 mm)4.0 in (102 mm)-2.0 in (-51 mm)-33.22%
Wheel (rim) diameter17.0 in (432 mm)21.0 in (533 mm)+4.0 in (+101 mm)+23.53%
Overall diameter29.0 in (737 mm)29.0 in (737 mm)0.0 in (0 mm)+0.01%
Circumference91.2 in (2316 mm)91.2 in (2317 mm)0.0 in (+1 mm)+0.01%
Revolutions per mile6956950-0.01%

Differences are 255/40R21 minus 235/65R17; percentages are relative to 235/65R17. These are nominal design dimensions. Real tires vary by brand, tread depth, wheel width, load and pressure.

The overall diameter differs by 0.01%, 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 235/65R17, the actual speed on 255/40R21 is about 60.0 mph. When it reads 60 mph on a vehicle built for 255/40R21, the actual speed on 235/65R17 is about 60.0 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, 235/65R17 against 255/40R21
Speedometer readsActual on 255/40R21 (built for 235/65R17)Actual on 235/65R17 (built for 255/40R21)
20 mph20.0 mph20.0 mph
30 mph30.0 mph30.0 mph
40 mph40.0 mph40.0 mph
50 mph50.0 mph50.0 mph
60 mph60.0 mph60.0 mph
70 mph70.0 mph70.0 mph
80 mph80.0 mph80.0 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.

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Shop for 235/65R17 or 255/40R21 tires

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Other comparisons with 235/65R17

Side-by-side dimensions and speedometer difference for sizes close to 235/65R17 in overall diameter.

Other comparisons with 255/40R21

Side-by-side dimensions and speedometer difference for sizes close to 255/40R21 in overall diameter.

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