285/50R20 vs 245/75R17
245/75R17 is 0.8% taller (+0.25 in) and 1.57 in narrower than 285/50R20.
245/75R17 is 0.8% taller than 285/50R20. That is inside the ±3% most shops treat as a straight swap.
Real speed on 245/75R17.
Change in height under the axle.
On the same wheel, split inboard and outboard.
Shop 245/75R17 tires
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285/50R20 or 245/75R17?
Can I use 245/75R17 instead of 285/50R20?
245/75R17 is 0.8% taller than 285/50R20. 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 245/75R17 you are really doing 60.5 mph if the speedometer was set up for 285/50R20. Going the other way, 285/50R20 would make it read 59.5 mph at a real 60.
How much ground clearance does it change?
Half the diameter difference: +0.12 in (+3 mm) under the axle.