This can be a nice math exercise 
Ratio of each gear is:
Tire RPM / Engine RPM.
To calculate Tire RPM, you first need the tire circumference. Lets say that you have 245/40x18 tire.
Circumference of the tire in meters is calculated as:
(tireWidth * tireAspect / 100 + wheelDiameter * 25.4 * 0.5) * 2 * PI / 1000;
So 245/40x18 tire circumference would be:
(245 * 40 / 100 + 18 * 25.4 * 0.5) * 2 * PI / 1000 = 2.05 meters
Now we have the circumference, we can calculate the tire RPM based on circumference and speed.
Speed in meters per minute is:
V (in km/h) * 1000 (meters in a km) / 60 (minutes in an hour)
So Tire RPM is:
TireRPM = CurrentSpeedKPH * 1000 / (60 * TireCircumference);
The current gear ratio is then simply Engine RPM / TireRPM