The formula
mph = rpm × tire diameter (in) ÷ (gear ratio × final drive × 336). The 336 converts inches per minute into miles per hour (63,360 inches in a mile, 60 minutes in an hour, and π for the tire's circumference). Flip it around to get RPM at a given speed.
Shorter vs taller final drive
A numerically higher final drive (say 4.10 → 4.30) is "shorter". It multiplies torque more, so the car pulls harder in every gear, but each gear tops out at a lower speed and the engine spins faster on the highway. A lower number is "taller": calmer cruising, slower acceleration. Run both through the calculator to see exactly where your shift points land.
Watch the RPM after each upshift
If an upshift at redline drops you below the engine's power band, the car bogs. Track and autocross drivers look at this column to decide which gear to hold through a corner and whether a different final drive keeps them in the meat of the power band.
Changing tire size changes your gearing
A taller tire acts like a taller final drive. Going about 3% taller in diameter is roughly the same as going 3% lower on the final drive ratio. Use the tire size calculator to compare sizes first.