225/70R21 vs 225/65R21
Going from one to the other changes the overall height by -2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/70R21 | 225/65R21 | Change |
|---|---|---|---|
| Overall height | 33.4" | 32.5" | -2.7 % |
| Sidewall | 6.20" | 5.76" | -7.1 % |
| Section width | 225 mm | 225 mm | 0.0 % |
| Circumference | 104.9" | 102.2" | -2.7 % |
| Revs per mile | 604 | 620 | +2.7 % |
| Axle height | 16.70" | 16.26" | -11.3 mm |
| Approved rim width | 6–8.5" | 6–8.5" | overlap 6–8.5" |
Both are approved on a rim 6" 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 225/70R21. Fit 225/65R21 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 | 68.1 mph | -1.9 |
| 90 mph | 87.6 mph | -2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,735 on the road, so it over-counts by 265. At a true 70 mph the first turns 704 times a minute, the second 724.