295/35R20 vs 255/55R18
New overall diameter
29.04″
was 28.13″
True speed at an indicated 60
61.9 mph
your speedo reads low
Revolutions per mile
694
was 717, −23
| Measurement | 295/35R20 | 255/55R18 | Change |
|---|---|---|---|
| Overall diameter | 28.13″ | 29.04″ | +0.91″ (+3.2%) |
| Sidewall height | 4.06″ (103.3 mm) | 5.52″ (140.3 mm) | +37.0 mm |
| Tread width | 295 mm | 255 mm | −40 mm |
| Circumference | 88.37″ | 91.24″ | +2.87″ |
| Revs per mile | 717 | 694 | −23 |
| Speedometer error | baseline | +3.2% | reads low |
Diameter overlay
295/35R20 255/55R18
Speedometer check
With 255/55R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.8 mph actual
35 mph shown
36.1 mph actual
45 mph shown
46.5 mph actual
55 mph shown
56.8 mph actual
65 mph shown
67.1 mph actual
75 mph shown
77.4 mph actual
Common questions
- Will 255/55R18 fit instead of 295/35R20?
- Overall diameter changes 3.2%; section width changes -40 mm. Diameter comparison alone cannot confirm fitment. Verify the vehicle maker's approved rim width, tire load and speed ratings, and clearance at full steering lock and suspension compression. Confirm any possible ABS, traction-control, speedometer, and drivetrain effects with the vehicle maker or a qualified tire professional. Overall diameter goes from 28.13″ to 29.04″, a change of +3.2%. 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 255/55R18?
- At an indicated 60 mph you are really doing about 61.9 mph, a +3.2% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 295/35R20 and 255/55R18?
- 295/35R20 turns about 717 times per mile and 255/55R18 turns about 694, a difference of −23. Your speedometer and odometer both count rotations, so this number is what throws them off.