225/70R17.5 vs 205/80R17.5
Going from one to the other changes the overall height by +1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/70R17.5 | 205/80R17.5 | Change |
|---|---|---|---|
| Overall height | 29.9" | 30.4" | +1.7 % |
| Sidewall | 6.20" | 6.46" | +4.1 % |
| Section width | 225 mm | 205 mm | -8.9 % |
| Circumference | 93.9" | 95.5" | +1.7 % |
| Revs per mile | 674 | 663 | -1.7 % |
| Axle height | 14.95" | 15.21" | +6.5 mm |
| Approved rim width | 6–8.5" | 5.5–7.5" | overlap 6–7.5" |
Both are approved on a rim 6" to 7.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 225/70R17.5. Fit 205/80R17.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.2 mph | +1.2 |
| 90 mph | 91.5 mph | +1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,171 on the road, so it under-counts by 171. At a true 70 mph the first turns 787 times a minute, the second 774.