235/70R17 vs 255/50R20
255/50R20 is 0.3% taller (+0.09 in) and 0.79 in wider than 235/70R17.
255/50R20 is 0.3% taller than 235/70R17. That is inside the ±3% most shops treat as a straight swap.
Real speed on 255/50R20.
Change in height under the axle.
On the same wheel, split inboard and outboard.
Shop 255/50R20 tires
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235/70R17 or 255/50R20?
Can I use 255/50R20 instead of 235/70R17?
255/50R20 is 0.3% taller than 235/70R17. 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 255/50R20 you are really doing 60.2 mph if the speedometer was set up for 235/70R17. Going the other way, 235/70R17 would make it read 59.8 mph at a real 60.
How much ground clearance does it change?
Half the diameter difference: +0.04 in (+1 mm) under the axle.