275/60R18 vs 305/60R18
New overall diameter
32.41″
was 30.99″
True speed at an indicated 60
62.7 mph
your speedo reads low
Revolutions per mile
622
was 651, −28
| Measurement | 275/60R18 | 305/60R18 | Change |
|---|---|---|---|
| Overall diameter | 30.99″ | 32.41″ | +1.42″ (+4.6%) |
| Sidewall height | 6.50″ (165.0 mm) | 7.20″ (183.0 mm) | +18.0 mm |
| Tread width | 275 mm | 305 mm | +30 mm |
| Circumference | 97.36″ | 101.82″ | +4.45″ |
| Revs per mile | 651 | 622 | −28 |
| Speedometer error | baseline | +4.6% | reads low |
Diameter overlay
275/60R18 305/60R18
Speedometer check
With 305/60R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
26.1 mph actual
35 mph shown
36.6 mph actual
45 mph shown
47.1 mph actual
55 mph shown
57.5 mph actual
65 mph shown
68.0 mph actual
75 mph shown
78.4 mph actual
Common questions
- Will 305/60R18 fit instead of 275/60R18?
- At 4.6% this lands between the 3% and 5% marks. Plenty of people run swaps like this, but you will see real speedometer error, and you want to check the wheel well at full lock and full compression before you buy. Your speedometer will read low, so you are going faster than it says. Overall diameter goes from 30.99″ to 32.41″, a change of +4.6%. 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 305/60R18?
- At an indicated 60 mph you are really doing about 62.7 mph, a +4.6% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 275/60R18 and 305/60R18?
- 275/60R18 turns about 651 times per mile and 305/60R18 turns about 622, a difference of −28. Your speedometer and odometer both count rotations, so this number is what throws them off.