A tire size comparison calculator is two stamps and the difference between them. The tool above starts with 275/65R20 against 315/70R17. The muted outline is the current size. The orange outline is the new one. Diameter percent is the speedometer error.
What to look at, in order
- Diameter percent. Stay inside about 3% if you want the speedometer and the well to stay in the same conversation. Past 5% you are changing gearing in a way you will feel.
- Width percent. A much wider tire needs a wider wheel and more room in the well. Diameter can match while width does not.
- The 60 mph line. That is the speedometer error in a unit you already use. A +3% diameter at 60 indicated is 61.8 mph actual.
The tire size chart is the lookup when you do not have the second stamp yet. This page is the live tool. The homepage calculator is the same engine with the compare toggle closed by default.
A worked comparison
| Measurement | 275/65R20 | 315/70R17 | Diff |
|---|---|---|---|
| Diameter | 34.1 in | 34.4 in | +0.8% |
| Width | 10.8 in | 12.4 in | +14.5% |
| Sidewall | 7.0 in | 8.7 in | |
| At 60 indicated | 60 mph | 60.5 mph actual |
Diameter is almost a wash. Width is not. This swap is a well-and-wheel problem, not a speedometer problem. That is the usual finding when someone "just wants 35s" and lands on a 315 that is still 34.4 inches tall. A true 35 is 35x12.50R17.
Tire comparison as a shopping filter
Retailers rank for these queries because the searcher is about to buy. Use the comparison to throw out sizes that grow too much, then take the shortlist to a shop that has done your truck. We will not tell you the 315 clears a stock F-150. We will tell you it is an inch and a half wider than the 275 it would replace.