Tire Size Calculator — Speedo Error + Gearing
Old vs new rubber: diameter change, true speed at 65 indicated, effective axle ratio.
DIAMETER CHANGE
—
Old / new diameter—
True speed at 65 indicated—
Effective axle ratio—
Worked example: 295/75R22.5 → 285/75R22.5, 3.42 axle
Diameter 39.92 → 39.33 in (−1.48%). Indicated 65 mph is 64.04 true; effective ratio 3.47.
Tire questions, answered
How much does a tire size change my speedometer?
By the diameter ratio: 295/75R22.5 to 285/75R22.5 shrinks about 1.5%, so an indicated 65 mph is about 64.0 true — and the odometer over-reads miles by the same share.
Do bigger tires change my gear ratio?
Effectively, yes: taller tires lower cruise RPM like a taller gear. A 3.42 axle with ~1.2% taller rubber behaves like ~3.38 at the same road speed.
Will new tire size change my MPG math?
Slightly — uncorrected odometers misstate MPG by the same % as the speedo. Fix the calibration first, then re-run your cost per mile fuel line.