255/40R21 vs 255/45R20
255/45R20 is 0.0% taller (+0.00 in) and the same width than 255/40R21.
255/45R20 and 255/40R21 are the same overall diameter, so the speedometer stays accurate.
Real speed on 255/45R20.
Change in height under the axle.
On the same wheel, split inboard and outboard.
Shop 255/45R20 tires
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255/40R21 or 255/45R20?
Can I use 255/45R20 instead of 255/40R21?
255/45R20 and 255/40R21 are the same overall diameter, so the speedometer stays accurate. Also match or beat the original load index and speed rating, and check that the wheel width suits the new tire.
Will my speedometer be off?
At an indicated 60 mph on 255/45R20 you are really doing 60.0 mph if the speedometer was set up for 255/40R21. Going the other way, 255/40R21 would make it read 60.0 mph at a real 60.
How much ground clearance does it change?
Half the diameter difference: +0.00 in (+0 mm) under the axle.