275/70R18 vs 255/70R18
New overall diameter
32.06″
was 33.16″
True speed at an indicated 60
58.0 mph
your speedo reads high
Revolutions per mile
629
was 608, +21
| Measurement | 275/70R18 | 255/70R18 | Change |
|---|---|---|---|
| Overall diameter | 33.16″ | 32.06″ | −1.10″ (−3.3%) |
| Sidewall height | 7.58″ (192.5 mm) | 7.03″ (178.5 mm) | −14.0 mm |
| Tread width | 275 mm | 255 mm | −20 mm |
| Circumference | 104.17″ | 100.70″ | −3.46″ |
| Revs per mile | 608 | 629 | +21 |
| Speedometer error | baseline | −3.3% | reads high |
Diameter overlay
275/70R18 255/70R18
Speedometer check
With 255/70R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.2 mph actual
35 mph shown
33.8 mph actual
45 mph shown
43.5 mph actual
55 mph shown
53.2 mph actual
65 mph shown
62.8 mph actual
75 mph shown
72.5 mph actual
Common questions
- Will 255/70R18 fit instead of 275/70R18?
- At 3.3% 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 high, so you are going slower than it says. Overall diameter goes from 33.16″ to 32.06″, a change of −3.3%. 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 255/70R18?
- At an indicated 60 mph you are really doing about 58.0 mph, a −3.3% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 275/70R18 and 255/70R18?
- 275/70R18 turns about 608 times per mile and 255/70R18 turns about 629, a difference of +21. Your speedometer and odometer both count rotations, so this number is what throws them off.