245/70R21 vs 225/80R21
Going from one to the other changes the overall height by +1.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/70R21 | 225/80R21 | Change |
|---|---|---|---|
| Overall height | 34.5" | 35.2" | +1.9 % |
| Sidewall | 6.75" | 7.09" | +5.0 % |
| Section width | 245 mm | 225 mm | -8.2 % |
| Circumference | 108.4" | 110.5" | +1.9 % |
| Revs per mile | 585 | 573 | -1.9 % |
| Axle height | 17.25" | 17.59" | +8.5 mm |
| Approved rim width | 7–9" | 6–8.5" | 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 245/70R21. Fit 225/80R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.7 mph | +1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,194 on the road, so it under-counts by 194. At a true 70 mph the first turns 682 times a minute, the second 669.