Tire Size Box

255/35R20 vs 255/45R20

255/45R20 compared with 255/35R20

+7.4% diameter, 0.0" width

255/45R20 is 29.04" tall vs 27.03". More than the 3% rule of thumb, so check carefully before switching.

27.0"20" rim3.5"10.0" wide
255/35R20 (solid) compared with 255/45R20 (dashed outline), drawn to scale

With 255/45R20 tires, when your speedometer says 60 mph you are really going

64.5 mph

255/45R20 is +2.01 inches (+7.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/35R20255/45R20Difference%
Overall diameter27.03"29.04"+2.01"+7.4%
Section width10.04"10.04"0.00"0.0%
Sidewall height3.51"4.52"+1.00"+28.6%
Circumference84.91"91.22"+6.31"+7.4%
Revolutions per mile746.2694.6−51.6−6.9%

Speedometer difference

If your vehicle came with 255/35R20 and you fit 255/45R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.5 mph+1.5 mph
30 mph32.2 mph+2.2 mph
40 mph43.0 mph+3.0 mph
50 mph53.7 mph+3.7 mph
60 mph64.5 mph+4.5 mph
65 mph69.8 mph+4.8 mph
70 mph75.2 mph+5.2 mph
75 mph80.6 mph+5.6 mph
80 mph85.9 mph+5.9 mph

Each size on its own

Questions

Which is bigger, 255/35R20 or 255/45R20?
255/45R20 is taller by 2.01 inches (29.04" vs 27.03"). 255/35R20 is the same width.
Can I replace 255/35R20 with 255/45R20?
It is not recommended without checking carefully. The diameter difference is +7.4%, more than the 3% most shops and makers allow. It will throw off the speedometer and may affect ABS, stability control and clearance.
How much will 255/45R20 change my speedometer?
When your speedometer reads 60 mph with 255/45R20 fitted (calibrated for 255/35R20), your real speed is 64.5 mph. Your odometer will under-count miles by about 7.4%.
Will 255/45R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.00 inches of ground clearance at the axle.

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