There's a long way to explain gear ratios, but let's try a shorter approach.
A gear ratio is the ratio between the input shaft, and the output shaft. The input shaft would be the crankshaft correct me if i'm wrong anyone, and the output shaft is the gears in the gearbox. The way the gear ratio is listed out is:
Number of times the input shaft spins or engine spins : Number of times the output shaft * or transmission* spins
In the case of the city you've shown,
First gear- 3.5 : 1 (engine spins 3.5 times for every one revolution of the transmission)
Second gear- 1.9 : 1 (engine spins 1.9 times for every one revolution of the transmission)
Third gear- 1.3 : 1 (engine sipns 1.3 times for every one revolution of the transmission)
Fourth gear ratio in most 5 speed transmissions is as close to 1 : 1 as possible, and there the torque generated from the gearbox and the engine is equal. And in fifth gear, it's going to be below 1 for the input shaft (that's why you get good fuel consumption in fifth gear)
There's a mathematical way of getting gear ratio. I think it was dividing the number of teeth in the input shaft by the number in the output shaft. Terms used are "ring and pinion". I don't quite know which one is the ring, and which one is the pinion. Or maybe this was for the final drive ratio... :S
I think i've gotten everything...