33x12.50R15 vs 305/55R20
305/55R20 is 0.6% taller (+0.21 in) and 0.49 in narrower than 33x12.50R15.
305/55R20 is 0.6% taller than 33x12.50R15. That is inside the ±3% most shops treat as a straight swap.
| 33x12.50R15 | 305/55R20 | Change | |
|---|---|---|---|
| Diameter | 33.00″ | 33.21″ | +0.21″+0.6% |
| Width | 12.50″ | 12.01″ | −0.49″−3.9% |
| Sidewall | 9.00″ | 6.60″ | −2.40″−26.6% |
| Circumference | 103.7″ | 104.3″ | +0.7″+0.6% |
| Wheel | 15″ | 20″ | +5″ |
| Revs per mile | 611 | 607 | −4 |
Real speed on 305/55R20.
Change in height under the axle.
On the same wheel, split inboard and outboard.
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33x12.50R15 or 305/55R20?
Can I use 305/55R20 instead of 33x12.50R15?
305/55R20 is 0.6% taller than 33x12.50R15. 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 305/55R20 you are really doing 60.4 mph if the speedometer was set up for 33x12.50R15. Going the other way, 33x12.50R15 would make it read 59.6 mph at a real 60.
How much ground clearance does it change?
Half the diameter difference: +0.10 in (+3 mm) under the axle.