275/75R21 vs 275/70R21
Going from one to the other changes the overall height by -2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/75R21 | 275/70R21 | Change |
|---|---|---|---|
| Overall height | 37.2" | 36.2" | -2.9 % |
| Sidewall | 8.12" | 7.58" | -6.7 % |
| Section width | 275 mm | 275 mm | 0.0 % |
| Circumference | 117.0" | 113.6" | -2.9 % |
| Revs per mile | 542 | 558 | +2.9 % |
| Axle height | 18.62" | 18.08" | -13.8 mm |
| Approved rim width | 7.5–10" | 7.5–10" | overlap 7.5–10" |
Both are approved on a rim 7.5" to 10" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 275/75R21. Fit 275/70R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.4 mph | -2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,709 on the road, so it over-counts by 291. At a true 70 mph the first turns 632 times a minute, the second 651.