A tire size speedometer calculator only cares about overall diameter. The cluster counts wheel revolutions and assumes the stock circumference. A taller tire travels farther per revolution, so the cluster under-reads actual speed and distance by the same percent the diameter grew. The compare tool above is already open on 275/65R20 vs 315/70R17. The 60 mph line is this page for that pair.
The formula
Error % = (new diameter - stock diameter) / stock diameter × 100. Actual speed = indicated × (1 + error/100). A 5% taller tire at 60 indicated is 63 mph actual. A 5% shorter tire at 60 indicated is 57 mph actual.
| Diameter change | Actual speed | After 100 indicated miles |
|---|---|---|
| -3% | 58.2 mph | 97.0 mi actual |
| 0% | 60.0 mph | 100 mi actual |
| +3% | 61.8 mph | 103.0 mi actual |
| +6% | 63.6 mph | 106.0 mi actual |
Why the odometer is in the same boat
Distance uses the same circumference assumption. A taller tire makes the odometer under-read. That matters for service intervals, leases and any math you do off the dash. GPS speed is the check: it does not care what is on the hubs.
Put the door-jamb size in A and the new size in B on the comparison. The 60 mph line is this table for your pair. The 33 vs 35 page is the 6% case people actually make.
Many trucks can have the tire diameter written into the BCM. That is the fix. Guessing a new speedo gear is the old fix. Leaving it is how you collect tickets and miss service.