How it's calculated
Your wheels turn slower than the engine by the overall ratio (gear × final drive). Multiply wheel revs by the tire's circumference and you get road speed:
speed = (rpm ÷ (gear × final)) × π × tireDiameter
That's why a numerically lower final drive (e.g. 3.5 vs 3.9) gives higher top speed but lazier acceleration, and a taller tire raises your speed at any given RPM.
FAQ
What gear ratio do I enter?
The individual gear you care about — e.g. 5th might be 0.80, 1st might be 3.5. Use 1.00 for a direct-drive top gear.
Where do I find my final drive?
It's the differential/axle ratio, often on a sticker, in the manual, or by VIN. Common road cars run 3.4–4.4.