215/45R18 vs 255/40R18
255/40R18 is 1.6% taller (+0.41 in) and 1.57 in wider than 215/45R18.
255/40R18 is 1.6% taller than 215/45R18. That is inside the ±3% most shops treat as a straight swap.
Real speed on 255/40R18.
Change in height under the axle.
On the same wheel, split inboard and outboard.
Shop 255/40R18 tires
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215/45R18 or 255/40R18?
Can I use 255/40R18 instead of 215/45R18?
255/40R18 is 1.6% taller than 215/45R18. 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 255/40R18 you are really doing 61.0 mph if the speedometer was set up for 215/45R18. Going the other way, 215/45R18 would make it read 59.0 mph at a real 60.
How much ground clearance does it change?
Half the diameter difference: +0.21 in (+5 mm) under the axle.