235/80R20 vs 245/80R20
Going from one to the other changes the overall height by +1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/80R20 | 245/80R20 | Change |
|---|---|---|---|
| Overall height | 34.8" | 35.4" | +1.8 % |
| Sidewall | 7.40" | 7.72" | +4.3 % |
| Section width | 235 mm | 245 mm | +4.3 % |
| Circumference | 109.3" | 111.3" | +1.8 % |
| Revs per mile | 579 | 569 | -1.8 % |
| Axle height | 17.40" | 17.72" | +8.0 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 235/80R20. Fit 245/80R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.3 mph | +1.3 |
| 90 mph | 91.6 mph | +1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,181 on the road, so it under-counts by 181. At a true 70 mph the first turns 676 times a minute, the second 664.