245/55R21 vs 265/50R21
Going from one to the other changes the overall height by -0.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/55R21 | 265/50R21 | Change |
|---|---|---|---|
| Overall height | 31.6" | 31.4" | -0.6 % |
| Sidewall | 5.31" | 5.22" | -1.7 % |
| Section width | 245 mm | 265 mm | +8.2 % |
| Circumference | 99.3" | 98.7" | -0.6 % |
| Revs per mile | 638 | 642 | +0.6 % |
| Axle height | 15.81" | 15.72" | -2.3 mm |
| Approved rim width | 7–9" | 7.5–9.5" | overlap 7.5–9" |
Both are approved on a rim 7.5" to 9" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 245/55R21. Fit 265/50R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.7 mph | -0.3 |
| 70 mph | 69.6 mph | -0.4 |
| 90 mph | 89.5 mph | -0.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,944 on the road, so it over-counts by 56. At a true 70 mph the first turns 744 times a minute, the second 749.