285/35R21 vs 255/40R21
255/40R21 is 0.6% taller (+0.18 in) and 1.18 in narrower than 285/35R21.
255/40R21 is 0.6% taller than 285/35R21. That is inside the ±3% most shops treat as a straight swap.
Real speed on 255/40R21.
Change in height under the axle.
On the same wheel, split inboard and outboard.
Shop 255/40R21 tires
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285/35R21 or 255/40R21?
Can I use 255/40R21 instead of 285/35R21?
255/40R21 is 0.6% taller than 285/35R21. That is inside the ±3% most shops treat as a straight swap. 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 255/40R21 you are really doing 60.4 mph if the speedometer was set up for 285/35R21. Going the other way, 285/35R21 would make it read 59.6 mph at a real 60.
How much ground clearance does it change?
Half the diameter difference: +0.09 in (+2 mm) under the axle.