Size comparison
| Spec | 315/70R17 | 35X12.50R18 | Change |
|---|---|---|---|
| Overall diameter | 34.4" | 35.0" | +0.6" |
| Section width | 12.4" | 12.5" | 0.1" |
| Rim diameter | 17" | 18" | 1" |
| Revolutions per mile | 587 | 576 | -11 |
| Ride height change | — | — | +0.3" |
Speedometer error
With a speedometer still calibrated for the 315/70R17, the taller 35X12.50R18 makes the needle read 1.9% low. Here is your real speed at common indicated readings:
| Speedometer shows | You are actually going |
|---|---|
| 60 mph | 61.1 mph |
| 65 mph | 66.2 mph |
| 70 mph | 71.3 mph |
The change is under 3%, so most trucks read close enough that recalibration is optional, though your odometer will be off by the same amount.
Gearing effect
Swapping to the taller 35X12.50R18 acts like a numerically lower axle gear — it drops your RPM at speed and softens acceleration. On a common 3.55 axle, your effective gearing becomes about 3.49.
To get your original power and RPM back on a 35X12.50R18, you would re-gear to about 3.62 — the closest common axle ratio is 3.55. Plan it exactly with the gear ratio calculator.
Run your own numbers
Different axle ratio, or want to test another size? The tire size calculator compares any two tires for diameter, speedometer error and gearing, and the gear ratio calculator dials in the axle ratio to match.