235/55R20 vs 245/55R20
Going from one to the other changes the overall height by +1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/55R20 | 245/55R20 | Change |
|---|---|---|---|
| Overall height | 30.2" | 30.6" | +1.4 % |
| Sidewall | 5.09" | 5.31" | +4.3 % |
| Section width | 235 mm | 245 mm | +4.3 % |
| Circumference | 94.8" | 96.2" | +1.4 % |
| Revs per mile | 668 | 659 | -1.4 % |
| Axle height | 15.09" | 15.31" | +5.5 mm |
| Approved rim width | 6.5–8.5" | 7–9" | overlap 7–8.5" |
Both are approved on a rim 7" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 235/55R20. Fit 245/55R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.3 mph | +1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,144 on the road, so it under-counts by 144. At a true 70 mph the first turns 780 times a minute, the second 769.