235/85R15 vs 245/85R15
Going from one to the other changes the overall height by +2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/85R15 | 245/85R15 | Change |
|---|---|---|---|
| Overall height | 30.7" | 31.4" | +2.2 % |
| Sidewall | 7.86" | 8.20" | +4.3 % |
| Section width | 235 mm | 245 mm | +4.3 % |
| Circumference | 96.5" | 98.6" | +2.2 % |
| Revs per mile | 656 | 642 | -2.2 % |
| Axle height | 15.36" | 15.70" | +8.5 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 235/85R15. Fit 245/85R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.1 mph | +1.1 |
| 70 mph | 71.5 mph | +1.5 |
| 90 mph | 92.0 mph | +2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,218 on the road, so it under-counts by 218. At a true 70 mph the first turns 766 times a minute, the second 749.