245/75R23 vs 245/70R23
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 | 245/75R23 | 245/70R23 | Change |
|---|---|---|---|
| Overall height | 37.5" | 36.5" | -2.6 % |
| Sidewall | 7.23" | 6.75" | -6.7 % |
| Section width | 245 mm | 245 mm | 0.0 % |
| Circumference | 117.7" | 114.7" | -2.6 % |
| Revs per mile | 538 | 552 | +2.6 % |
| Axle height | 18.73" | 18.25" | -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 245/75R23. Fit 245/70R23 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 9,743 on the road, so it over-counts by 257. At a true 70 mph the first turns 628 times a minute, the second 645.