315/75R16 vs 285/75R18
285/75R18 is 0.7% taller (+0.23 in) and 1.18 in narrower than 315/75R16.
285/75R18 is 0.7% taller than 315/75R16. That is inside the ±3% most shops treat as a straight swap.
Real speed on 285/75R18.
Change in height under the axle.
On the same wheel, split inboard and outboard.
Shop 285/75R18 tires
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315/75R16 or 285/75R18?
Can I use 285/75R18 instead of 315/75R16?
285/75R18 is 0.7% taller than 315/75R16. 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 285/75R18 you are really doing 60.4 mph if the speedometer was set up for 315/75R16. Going the other way, 315/75R16 would make it read 59.6 mph at a real 60.
How much ground clearance does it change?
Half the diameter difference: +0.11 in (+3 mm) under the axle.