245/80R21 vs 235/85R21
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 | 245/80R21 | 235/85R21 | Change |
|---|---|---|---|
| Overall height | 36.4" | 36.7" | +0.8 % |
| Sidewall | 7.72" | 7.86" | +1.9 % |
| Section width | 245 mm | 235 mm | -4.1 % |
| Circumference | 114.5" | 115.4" | +0.8 % |
| Revs per mile | 554 | 549 | -0.8 % |
| Axle height | 18.22" | 18.36" | +3.8 mm |
| Approved rim width | 7–9" | 6.5–8.5" | overlap 7–8.5" |
Both are approved on a rim 7" 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 245/80R21. Fit 235/85R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.4 mph | +0.4 |
| 70 mph | 70.6 mph | +0.6 |
| 90 mph | 90.7 mph | +0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,081 on the road, so it under-counts by 81. At a true 70 mph the first turns 646 times a minute, the second 641.