235/75R21 vs 225/80R21
Going from one to the other changes the overall height by +0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/75R21 | 225/80R21 | Change |
|---|---|---|---|
| Overall height | 34.9" | 35.2" | +0.8 % |
| Sidewall | 6.94" | 7.09" | +2.1 % |
| Section width | 235 mm | 225 mm | -4.3 % |
| Circumference | 109.6" | 110.5" | +0.8 % |
| Revs per mile | 578 | 573 | -0.8 % |
| Axle height | 17.44" | 17.59" | +3.8 mm |
| Approved rim width | 6.5–8.5" | 6–8.5" | overlap 6.5–8.5" |
Both are approved on a rim 6.5" 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 235/75R21. Fit 225/80R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.4 mph | +0.4 |
| 70 mph | 70.6 mph | +0.6 |
| 90 mph | 90.8 mph | +0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,085 on the road, so it under-counts by 85. At a true 70 mph the first turns 675 times a minute, the second 669.