245/45R18 vs 245/35R20
New overall diameter
26.75″
was 26.68″
True speed at an indicated 60
60.2 mph
your speedo reads low
Revolutions per mile
754
was 756, −2
| Measurement | 245/45R18 | 245/35R20 | Change |
|---|---|---|---|
| Overall diameter | 26.68″ | 26.75″ | +0.07″ (+0.3%) |
| Sidewall height | 4.34″ (110.3 mm) | 3.38″ (85.8 mm) | −24.5 mm |
| Tread width | 245 mm | 245 mm | +0 mm |
| Circumference | 83.82″ | 84.04″ | +0.22″ |
| Revs per mile | 756 | 754 | −2 |
| Speedometer error | baseline | +0.3% | reads low |
Diameter overlay
245/45R18 245/35R20
Speedometer check
With 245/35R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.1 mph actual
35 mph shown
35.1 mph actual
45 mph shown
45.1 mph actual
55 mph shown
55.1 mph actual
65 mph shown
65.2 mph actual
75 mph shown
75.2 mph actual
Common questions
- Will 245/35R20 fit instead of 245/45R18?
- Diameter moves 0.3%, 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 26.68″ to 26.75″, a change of +0.3%. 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 245/35R20?
- At an indicated 60 mph you are really doing about 60.2 mph, a +0.3% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 245/45R18 and 245/35R20?
- 245/45R18 turns about 756 times per mile and 245/35R20 turns about 754, a difference of −2. Your speedometer and odometer both count rotations, so this number is what throws them off.