235/40R18 vs 205/65R15
205/65R15 is 0.4% taller (+0.09 in) and 1.18 in narrower than 235/40R18.
205/65R15 is 0.4% taller than 235/40R18. That is inside the ±3% most shops treat as a straight swap.
Real speed on 205/65R15.
Change in height under the axle.
On the same wheel, split inboard and outboard.
Shop 205/65R15 tires
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235/40R18 or 205/65R15?
Can I use 205/65R15 instead of 235/40R18?
205/65R15 is 0.4% taller than 235/40R18. 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 205/65R15 you are really doing 60.2 mph if the speedometer was set up for 235/40R18. Going the other way, 235/40R18 would make it read 59.8 mph at a real 60.
How much ground clearance does it change?
Half the diameter difference: +0.05 in (+1 mm) under the axle.