245/85R21 vs 245/80R21
Going from one to the other changes the overall height by -2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/85R21 | 245/80R21 | Change |
|---|---|---|---|
| Overall height | 37.4" | 36.4" | -2.6 % |
| Sidewall | 8.20" | 7.72" | -5.9 % |
| Section width | 245 mm | 245 mm | 0.0 % |
| Circumference | 117.5" | 114.5" | -2.6 % |
| Revs per mile | 539 | 554 | +2.6 % |
| Axle height | 18.70" | 18.22" | -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 245/85R21. Fit 245/80R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,742 on the road, so it over-counts by 258. At a true 70 mph the first turns 629 times a minute, the second 646.