255/40R18 vs 245/45R18
245/45R18 is 2.5% taller (+0.65 in) and 0.39 in narrower than 255/40R18.
245/45R18 is 2.5% taller than 255/40R18. That is inside the ±3% most shops treat as a straight swap.
Real speed on 245/45R18.
Change in height under the axle.
On the same wheel, split inboard and outboard.
Shop 245/45R18 tires
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255/40R18 or 245/45R18?
Can I use 245/45R18 instead of 255/40R18?
245/45R18 is 2.5% taller than 255/40R18. 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 245/45R18 you are really doing 61.5 mph if the speedometer was set up for 255/40R18. Going the other way, 255/40R18 would make it read 58.5 mph at a real 60.
How much ground clearance does it change?
Half the diameter difference: +0.32 in (+8 mm) under the axle.