Skip to main content
TireShopper

255/40R20 vs 315/40R18 tire size comparison

255/40R20315/40R18

315/40R18 is 0.1 in (0.39%) shorter than 255/40R20 and 2.4 in wider. They fit different wheels: 255/40R20 a 20-inch wheel and 315/40R18 an 18-inch wheel.

255/40R20 and 315/40R18 side by side

-0.1 in / -0.39%255/40R20: 28.0 in tall, solid line315/40R18: 27.9 in tall, dashed line255/40R20Width 10.0 in (255 mm)Sidewall 4.0 in (102 mm)315/40R18Width 12.4 in (315 mm)Sidewall 5.0 in (126 mm)
255/40R20 is drawn with a solid line and 315/40R18 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 255/40R20 and 315/40R18, with the difference (315/40R18 minus 255/40R20)
Measurement255/40R20315/40R18DifferenceDifference %
Section width10.0 in (255 mm)12.4 in (315 mm)+2.4 in (+60 mm)+23.53%
Sidewall height4.0 in (102 mm)5.0 in (126 mm)+1.0 in (+24 mm)+23.53%
Wheel (rim) diameter20.0 in (508 mm)18.0 in (457 mm)-2.0 in (-51 mm)-10.00%
Overall diameter28.0 in (712 mm)27.9 in (709 mm)-0.1 in (-3 mm)-0.39%
Circumference88.1 in (2237 mm)87.7 in (2228 mm)-0.4 in (-9 mm)-0.39%
Revolutions per mile719722+3+0.39%

Differences are 315/40R18 minus 255/40R20; percentages are relative to 255/40R20. 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 255/40R20, the actual speed on 315/40R18 is about 59.8 mph. When it reads 60 mph on a vehicle built for 315/40R18, the actual speed on 255/40R20 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, 255/40R20 against 315/40R18
Speedometer readsActual on 315/40R18 (built for 255/40R20)Actual on 255/40R20 (built for 315/40R18)
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 255/40R20 or 315/40R18 tires

Confirm the size on your tire sidewall or door placard first. These links open each store's own search; prices and stock are on their sites.

We may earn a commission if you buy through these links. It doesn't change the price you pay. Read our affiliate disclosure.

Other comparisons with 255/40R20

Side-by-side dimensions and speedometer difference for sizes close to 255/40R20 in overall diameter.

Other comparisons with 315/40R18

Side-by-side dimensions and speedometer difference for sizes close to 315/40R18 in overall diameter.

You might also need

Paid links to Amazon search results. As an Amazon Associate I earn from qualifying purchases. Read our affiliate disclosure.