What is gear ratio?

In an interaction with an automobile engineer, i was informed that any manufacturer will look to 3 main aspects before producing or designing a vehicle. These 3 factors are,

1. Aero dynamic shape
2. Power to weight ratio
3. Gear ratio

Aero dynamic shape & power to weight ratio was little bit understood, but gear ratio is still unknown. Can anyone pls tell me abt gear ratio & its effects & advantages

Gear ratio is a ratio of number of teeth of the driven gear to that on the driving gear.

See this as an example:

Here the driver has 60 teeth and driven has 30. So the gear ratio is 3:60 i.e. 1:2.

In low gears, the driven gear almost has more teeth and in overdrive, the driven  gear has less number of teeth compared to the driving gear.

In addition to Rachit's post, this table will (hopefully) add more clarity:

In practice all gear positions except the 5^th correspond to the 1^st row in the above table - the input gear has less teeth than the output gear but as the gear position increases from 1 to 4, the no. of teeth in input gear become more in comparison to that of the output. The 5^th gear position has greater teeth on the input than the output - so this maximises speed transfer instead of power - great for cruising at high speeds.

In addition there's a final constant reduction in the differential (by design) - applicable at all gear positions.

In same cars, 4th gera is a direct drive (i.e. 2nd row). If it isn't then its almost close to one.

Great info this is. Thanks a lot guys for using easy language.

Sorry to say , m still not getting it. Can u pls tell me what is driving gear & what is driven gear.

This much i understood, 4 stroke engine, power transferred to crankshaft, and carck shaft connected to flywheel, flywheel with cluth. Now my question is then what? Whats the next steps?

Pls dont mind, i'll keep asking till doubt is cleared.

Well, from clutch it goes to the drive (input) gear (toothed wheel). This is coupled to the output (driven) gear (toothed wheel). Now don't confuse the word "gear" - it is used for both the actual toothed wheel as well as the position of the stick. The latter basically means that the gears have been engaged (or the process of doing it).

So on this basis, can i assume that drive gear is what connected with crankshaft (partially) and driven gear is connected with axle.

Yes.

Gear is a toothed wheel. Fine. For changing from neutral to 1 gear, then how many toothed wheel will it use, to transfer the generated power/torque to the wheel.

And what does it mean by synchromesh gear box?