Tire size comparison calculator
Will that size fit? Compare two sizes and see exactly what changes.
| Measurement | New | Difference |
|---|---|---|
| Overall diameter | 25.72 in | +0.74 in (+3.0%) |
| Section width | 9.65 in | +0.79 in |
| Sidewall height | 3.86 in | −0.13 in |
| Circumference | 80.79 in | +2.34 in |
| Revolutions per mile | 784 | −23 |
| Ride height change | +0.37 in | half the diameter change |
| Speedo shows | You are actually doing | Difference |
|---|---|---|
| 20 | 20.6 | +0.6 |
| 30 | 30.9 | +0.9 |
| 40 | 41.2 | +1.2 |
| 50 | 51.5 | +1.5 |
| 60 | 61.8 | +1.8 |
| 70 | 72.1 | +2.1 |
| 80 | 82.4 | +2.4 |
Compare any two tire sizes and see what changes: the diameter difference, the speedometer error it causes, the clearance it costs, and a to-scale drawing of both tires side by side.
Popular size changes
These are the stock-to-upgrade pairs people compare most often. Every figure below is computed from the same formulas the calculator uses, so you can check the math against your own numbers.
| Stock size | New size | New diameter | Difference | Speedo reads | Fitment |
|---|---|---|---|---|---|
| 225/45R17 | 245/40R18 | 25.72 in | +0.74 in (+3.0%) | 66.9 mph | Diameter: 3.0% larger |
| 205/55R16 | 225/45R17 | 24.97 in | +0.09 in (+0.4%) | 65.2 mph | Diameter: 0.4% larger |
| 235/40R18 | 245/35R19 | 25.75 in | +0.35 in (+1.4%) | 65.9 mph | Diameter: 1.4% larger |
| 265/70R17 | 285/70R17 | 32.71 in | +1.10 in (+3.5%) | 67.3 mph | Aggressive: 3.5% larger |
| 245/65R17 | 265/65R17 | 30.56 in | +1.02 in (+3.5%) | 67.3 mph | Aggressive: 3.5% larger |
Choose a bigger size from your stock size
The ±3% rule, and why it exists
When a new tire's overall diameter differs from the original by more than about three percent, three things change at once. The speedometer and odometer drift, because they are calibrated to a known number of wheel revolutions per mile. The effective gear ratio changes, which alters acceleration and can confuse an automatic transmission's shift points. And the tire's outer edge moves closer to the wheel arch and suspension, because half of any diameter increase goes upward into the arch.
Staying inside ±3% keeps all three effects small. Going beyond it is not automatically unsafe, and plenty of trucks run 5% larger tires, but you should expect a speedometer correction, a noticeable gearing change, and you should check clearance at full steering lock and full suspension compression.
What this calculator does not know
It compares published dimensions of new tires. Actual mounted dimensions vary between manufacturers, tread depth changes diameter slightly over a tire's life, and it says nothing about your specific vehicle's clearance, brake caliper clearance, load rating requirements or speedometer calibration. Treat the verdict as a filter, not an approval.