285/75R16 vs 305/70R17
New overall diameter
33.81″
was 32.83″
True speed at an indicated 60
61.8 mph
your speedo reads low
Revolutions per mile
596
was 614, −18
| Measurement | 285/75R16 | 305/70R17 | Change |
|---|---|---|---|
| Overall diameter | 32.83″ | 33.81″ | +0.98″ (+3.0%) |
| Sidewall height | 8.42″ (213.8 mm) | 8.41″ (213.5 mm) | −0.3 mm |
| Tread width | 285 mm | 305 mm | +20 mm |
| Circumference | 103.14″ | 106.22″ | +3.08″ |
| Revs per mile | 614 | 596 | −18 |
| Speedometer error | baseline | +3.0% | reads low |
Diameter overlay
285/75R16 305/70R17
Speedometer check
With 305/70R17 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.7 mph actual
35 mph shown
36.0 mph actual
45 mph shown
46.3 mph actual
55 mph shown
56.6 mph actual
65 mph shown
66.9 mph actual
75 mph shown
77.2 mph actual
Common questions
- Will 305/70R17 fit instead of 285/75R16?
- Diameter moves 3.0%, 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 low, so you are going faster than it says. Overall diameter goes from 32.83″ to 33.81″, a change of +3.0%. 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 305/70R17?
- At an indicated 60 mph you are really doing about 61.8 mph, a +3.0% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 285/75R16 and 305/70R17?
- 285/75R16 turns about 614 times per mile and 305/70R17 turns about 596, a difference of −18. Your speedometer and odometer both count rotations, so this number is what throws them off.