33x12.50R20 vs 305/55R20: full comparison
| Measurement | 33x12.50R20 | 305/55R20 | Difference |
|---|---|---|---|
| Diameter | 33.00 in | 33.21 in | +0.21 in+0.6% |
| Section width | 12.50 in | 12.01 in | −0.49 in−3.9% |
| Sidewall height | 6.50 in | 6.60 in | +0.10 in+1.6% |
| Circumference | 103.67 in | 104.33 in | +0.66 in+0.6% |
| Revs per mile | 611 | 607 | −4−0.6% |
Speedometer: indicated vs actual speed
With 305/55R20 fitted where 33x12.50R20 was the stock/calibrated size:
| Speedometer shows | Actual speed | Difference |
|---|---|---|
| 20 mph | 20.1 mph | +0.1 mph |
| 30 mph | 30.2 mph | +0.2 mph |
| 40 mph | 40.3 mph | +0.3 mph |
| 50 mph | 50.3 mph | +0.3 mph |
| 55 mph | 55.3 mph | +0.3 mph |
| 60 mph | 60.4 mph | +0.4 mph |
| 65 mph | 65.4 mph | +0.4 mph |
| 70 mph | 70.4 mph | +0.4 mph |
| 75 mph | 75.5 mph | +0.5 mph |
| 80 mph | 80.5 mph | +0.5 mph |
Recalculate for a gear ratio change too →
Will 305/55R20 fit where 33x12.50R20 was?
At +0.6%, 305/55R20 is within the ±3% most tire shops and manufacturers accept without recalibrating the speedometer or expecting rubbing -- it should generally bolt on, though wheel width, offset, and suspension travel still matter.
Related tire size comparisons
- 275/55R20 vs 33x12.50R20
- 275/60R20 vs 305/55R20
- 275/60R20 vs 33x12.50R20
- 295/65R20 vs 33x12.50R20
- 33x12.50R20 vs 285/55R20
More tire tools
- Tire size comparison tool: compare any two sizes yourself, including flotation sizes.
- Tire size calculator: full dimensions of one size, plus every equivalent size within 3%.
- Speedometer calculator: true speed after a tire or gear ratio change.
Frequently asked questions
How much bigger is 305/55R20 than 33x12.50R20?
305/55R20 is 0.21 in taller in overall diameter (+0.6%) and 0.49 in narrower in section width (−3.9%) than 33x12.50R20.
Will 305/55R20 fit in place of 33x12.50R20?
At +0.6%, 305/55R20 is within the ±3% most tire shops and manufacturers accept without recalibrating the speedometer or expecting rubbing -- it should generally bolt on, though wheel width, offset, and suspension travel still matter.
Does swapping to 305/55R20 affect my speedometer?
With 305/55R20 fitted where 33x12.50R20 was calibrated, an indicated 60 mph is really about 60.4 mph. That small a difference is within the ±3% most shops treat as close enough not to recalibrate.
Does 305/55R20 change the ride height?
Yes -- half the diameter difference shows up at the axle, so 305/55R20 changes ride height by about 0.10 in higher compared with 33x12.50R20.
Can I compare a metric size (like 305/55R20) with a flotation size (like 33x12.50R20)?
Yes -- both are converted to inches and millimeters before comparing, so the diameter, width and speedometer numbers above are directly comparable regardless of which sizing system each tire uses.