Congratulation on ur new purchase,sir.i hope u ll enjoy it more then ur last car 
the intake manifold can be bought easily but im afraid it ll be a bit expansive.u can guess the cost from the fact that civic 2001's intake manifold was for 14k PKR some time ago.but u can get it easily from any honda 3s dealer or may be even from open market.
The bar that u are talking about is the fuel economy meter.ur cars' display screen shows 2 types of fuel averages.one of the is the "Spontaneous" average.This is shown by the bar.This bar is always on the move, as u press the throttle/accelerate the car ,this bar reduces its length, showing that u are covering less KMs in 1 ltr fuel and as u release the accelerator (engine speed decreases) the bar extends itself towards right, showing a positive change in the fuel economy.
Exactly below it will be some numbers, showing the rear average of the distance covered.
u can reset it keeping the button pressed for 1second.then, when u drive ur car, it will calculate the average kms/ltr and will show a relatively stable figure.in short, the bar is for u to get used to optimum fuel economy (try to push it as much to the right as u can, for max economy) and the figure below it is the actual fuel average.
Hope u got the working
CNG is 100% successful in this city.my mechanic and friends (owner of new city) say its better on cng then its predecessor,the idsi.
for tuning, look into the main post of this thread.u r car is on petrol, that means it needs very less tuning compared to cng ones.
a tip for max fuel economy, try to shift gears before 2500rpm if u want max economy.
