235/55R18 vs 225/75R15
225/75R15 is 0.4% taller (+0.11 in) and 0.39 in narrower than 235/55R18.
225/75R15 is 0.4% taller than 235/55R18. That is inside the ±3% most shops treat as a straight swap.
Real speed on 225/75R15.
Change in height under the axle.
On the same wheel, split inboard and outboard.
Shop 225/75R15 tires
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235/55R18 or 225/75R15?
Can I use 225/75R15 instead of 235/55R18?
225/75R15 is 0.4% taller than 235/55R18. 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 225/75R15 you are really doing 60.2 mph if the speedometer was set up for 235/55R18. Going the other way, 235/55R18 would make it read 59.8 mph at a real 60.
How much ground clearance does it change?
Half the diameter difference: +0.06 in (+1 mm) under the axle.