The way real gears work.

A car engine cannot move the tires of a car from a dead stop at it's nomral power. So we reduce the gear ratio to help. 1:1 is normal, if we drop this to 2:1, the engine will turn two times for every time the wheel turns once. This means the car has twice the power, but will top out at a much lower high speed (the gears start much lower then this btw). For this reason we raise the ratio as the car builds up speed, and at high speed we can even go higher then 1:1 to help build up top speed or fuel economy.

Now that you know this you simply change the speed limit in phcon_setmotor (you used motor2 there) to a higher number as you change gear via script.

if(gear == 1)
{
motor2 = 20;
}
if(gear == 2)
{
motor2 = 30;
}

this should give you the idea