305/70R17 vs 285/75R16
New overall diameter
32.83″
was 33.81″
True speed at an indicated 60
58.3 mph
your speedo reads high
Revolutions per mile
614
was 596, +18
| Measurement | 305/70R17 | 285/75R16 | Change |
|---|---|---|---|
| Overall diameter | 33.81″ | 32.83″ | −0.98″ (−2.9%) |
| Sidewall height | 8.41″ (213.5 mm) | 8.42″ (213.8 mm) | +0.3 mm |
| Tread width | 305 mm | 285 mm | −20 mm |
| Circumference | 106.22″ | 103.14″ | −3.08″ |
| Revs per mile | 596 | 614 | +18 |
| Speedometer error | baseline | −2.9% | reads high |
Diameter overlay
305/70R17 285/75R16
Speedometer check
With 285/75R16 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.3 mph actual
35 mph shown
34.0 mph actual
45 mph shown
43.7 mph actual
55 mph shown
53.4 mph actual
65 mph shown
63.1 mph actual
75 mph shown
72.8 mph actual
Common questions
- Will 285/75R16 fit instead of 305/70R17?
- Diameter moves 2.9%, comfortably inside the 3% rule. Your speedometer, ABS, traction control and gearing all stay in the range they were calibrated for. Your speedometer will read high, so you are going slower than it says. Overall diameter goes from 33.81″ to 32.83″, a change of −2.9%. The rule of thumb most shops use: under 3% is fine, 3 to 5% needs a clearance check, over 5% needs recalibration.
- How far off will my speedometer read on 285/75R16?
- At an indicated 60 mph you are really doing about 58.3 mph, a −2.9% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 305/70R17 and 285/75R16?
- 305/70R17 turns about 596 times per mile and 285/75R16 turns about 614, a difference of +18. Your speedometer and odometer both count rotations, so this number is what throws them off.