Gear ratio is not exactly the same as gearbox ratio.
Gear ratio is the ratio between 2 gears. For example, between the pinion and the spur.
Gearbox ratio is aka transmission ratio. It is the final ratio after considering all the gear reduction taking place inside the gearbox or the transmission system. There can be more than 2 gears inside the gearbox. Note that normally when you calculate the ratios inside the gearbox, all the “middle gears” will cancel out themselves so you only look into the gear counts at the two ends.
To determine the ultimate total drive reduction of the whole drive train, you must multiply the transmission ratio by the pinion/spur gear ratio. The answer tells how many times the pinion must turn so to turn the diff gear (and the wheels) one complete revolution.
For simple 4WD shaft cars such as TT01 and TT02, the transmission ratio can be calculated by dividing the number of teeth on the differential’s ring gear by the number of teeth on the bevel pinion that it mates with. The differential’s ring gear is the larger one on the side, while the bevel pinion is the pinion attached to the main drive shaft.
Pan cars and most (but not all) F1 cars are direct-drive, meaning there are a pinion and a spur that also serves as a ball diff that holds the axle without other reduction. In such a case the transmission ratio is always 1 due to the fact that one revolution of “transmission” equals one revolution of the wheel.