Notice: this stuff only works in PC version and you must make sure that you are familiar with funky trees to follow this tutorial,you will also need some PID konwledge because I won't explain too much detials.
First you will need a "counter" function ,it will record the certain number of how many times you pressed a button ,I will give you a string of functions that you need.
//
round(sum(abs(rate(round(clamp01((Yaw=0?0:1)(Yaw>=0?1:-1))))))/2-sum(abs(rate(round(clamp((Yaw=0?0:1)(Yaw>=0?1:-1),-1,0)))))/2)
//
(use debugexpression and press Yaw to see what it can do.)
And about how to control the speed of your car ,one way is to use"PID"funtion,change the "target" of PID function by using the "counter",it will limit the max speed of your car and the maxspeed will be different when you are in different gears.
The "current" of PID will be the speed of your car(GS or GS*3.6).
"P" of the PID is a constant number,I use 0.04 but you may need test many times to find the number that apply your car.
And about the "I" of the PID,I recommend you leave it at zero.
finally the “D” parameter,also use the“counter” to set it to different number when you are in different gears,it will affect the Acceleration of your car.the bigger the number is ,the slower your car will accelerate.

This is the basic principle about sequential transmission,I'm afraid I cannot explain more becaues every car is different ,and this is just one way to do this.

If you can't understand this tutorial, I'm sorry for that , I'm a chinese and my english level is only enough for me to pass the final exam,if someone can understand,you can write a better one than this.