275/45R21 vs 235/75R17
235/75R17 is 0.4% taller (+0.13 in) and 1.57 in narrower than 275/45R21.
235/75R17 is 0.4% taller than 275/45R21. That is inside the ±3% most shops treat as a straight swap.
Real speed on 235/75R17.
Change in height under the axle.
On the same wheel, split inboard and outboard.
Shop 235/75R17 tires
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275/45R21 or 235/75R17?
Can I use 235/75R17 instead of 275/45R21?
235/75R17 is 0.4% taller than 275/45R21. 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 235/75R17 you are really doing 60.3 mph if the speedometer was set up for 275/45R21. Going the other way, 275/45R21 would make it read 59.7 mph at a real 60.
How much ground clearance does it change?
Half the diameter difference: +0.07 in (+2 mm) under the axle.