255/40R18 vs 235/45R18
235/45R18 is 1.1% taller (+0.30 in) and 0.79 in narrower than 255/40R18.
235/45R18 is 1.1% taller than 255/40R18. That is inside the ±3% most shops treat as a straight swap.
Real speed on 235/45R18.
Change in height under the axle.
On the same wheel, split inboard and outboard.
Shop 235/45R18 tires
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255/40R18 or 235/45R18?
Can I use 235/45R18 instead of 255/40R18?
235/45R18 is 1.1% 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 235/45R18 you are really doing 60.7 mph if the speedometer was set up for 255/40R18. Going the other way, 255/40R18 would make it read 59.3 mph at a real 60.
How much ground clearance does it change?
Half the diameter difference: +0.15 in (+4 mm) under the axle.