245/80R23 vs 245/85R23
Going from one to the other changes the overall height by +2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/80R23 | 245/85R23 | Change |
|---|---|---|---|
| Overall height | 38.4" | 39.4" | +2.5 % |
| Sidewall | 7.72" | 8.20" | +6.3 % |
| Section width | 245 mm | 245 mm | 0.0 % |
| Circumference | 120.7" | 123.8" | +2.5 % |
| Revs per mile | 525 | 512 | -2.5 % |
| Axle height | 19.22" | 19.70" | +12.3 mm |
| Approved rim width | 7–9" | 7–9" | overlap 7–9" |
Both are approved on a rim 7" to 9" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 245/80R23. Fit 245/85R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.3 mph | +2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,251 on the road, so it under-counts by 251. At a true 70 mph the first turns 612 times a minute, the second 597.