275/70R21 vs 275/75R21
Going from one to the other changes the overall height by +3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/70R21 | 275/75R21 | Change |
|---|---|---|---|
| Overall height | 36.2" | 37.2" | +3.0 % |
| Sidewall | 7.58" | 8.12" | +7.1 % |
| Section width | 275 mm | 275 mm | 0.0 % |
| Circumference | 113.6" | 117.0" | +3.0 % |
| Revs per mile | 558 | 542 | -3.0 % |
| Axle height | 18.08" | 18.62" | +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 275/70R21. Fit 275/75R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.5 mph | +1.5 |
| 70 mph | 72.1 mph | +2.1 |
| 90 mph | 92.7 mph | +2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,299 on the road, so it under-counts by 299. At a true 70 mph the first turns 651 times a minute, the second 632.