275/55R20 vs 275/65R18
275/65R18 is 0.5% taller (+0.17 in) and the same width than 275/55R20.
275/65R18 is 0.5% taller than 275/55R20. That is inside the ±3% most shops treat as a straight swap.
Real speed on 275/65R18.
Change in height under the axle.
On the same wheel, split inboard and outboard.
Shop 275/65R18 tires
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275/55R20 or 275/65R18?
Can I use 275/65R18 instead of 275/55R20?
275/65R18 is 0.5% taller than 275/55R20. 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 275/65R18 you are really doing 60.3 mph if the speedometer was set up for 275/55R20. Going the other way, 275/55R20 would make it read 59.7 mph at a real 60.
How much ground clearance does it change?
Half the diameter difference: +0.08 in (+2 mm) under the axle.