No problem. I shouldn't have gotten off-topic earlier.
The gear ratios will be different depending on the car. You can usually find them online if you want to run the speedometer/tachometer relationship experiment you described earlier.
Modern overdrive transmissions typically have gear ratios ranging from 5:1 in first gear to 0.75:1 in the overdrive gear (4,5,or even 8th gear in modern auto-transmissions).
The differential is the device that splits the torque between to two drive tires. Typical differential gear ratios for light-duty, rear wheel drive trucks is 3.77. This number will be higher for fuel-efficient, small engine, front wheel drive cars, like a Honda Accord.
Typically, car manufacture websites list the gear ratios in the transmission. The "final drive ratio" is the differential gear ratio.
To find the correlation between the vehicles speed and engine speed, we can use the following relationship.
\[v_c = {2 \pi r_t \times RPM * 2 \pi/60 \over gr_t \times gr_d}\]where \(r_t \) is the radius of your tires in meters, \(RPM\) is the engine speed in revolutions per minute, \(gr_t\) is the gear ratio of the current gear of the transmission, and \(gr_d\) is the gear ratio of the differential. \(v_c\) will be in units of m/s. Convert to other velocity units if desired.