I can understand ur state of mind......when i bought my vitz in 08...a friend of mine also bought the same vitz but it was 05 model (mine was 06) and it was 45 k kilometers run already n he was dam happy coz he got it for almost 75 thousand rs less. The difference became evident after 1 month...mine was as accurate as the dealer had stated....his was screwing up...same problems low pick and more consumption.
Then we started working on these problems and we found the solutions as well.
1) Your fuel pump assembly may be old n faulty now (the assembly contains fuel filter as well as pump) its located under the rear seat and in those days it costed 16.5k for a new one...dont know about now. When my friend again went to nathiagali on his car this time it wasnt giving the pick issues or consumptions issues any more.
2) Now i'll tell u one problem which arose in my car after a year of use. Its top speed was a bit choked...after crossing hundred it used to die down a little and consumption went slightly more....from 14 it came down to 12. So i changed my fuel filter (not the whole fuel pump assembly coz i got it checked n the expert told me that ur fuel pump is fine u just replace the fuel filter part in it) it costed me about 3200 rs in mid of 2009. Still the problem didnt really go away. So i went back to the mechanic n then he checked the CATALYTIC CONVERTER of my car (the front one)...it was partially choked....i got it broken but it was broken with a wrong technique which i realized later...he broke the whole of it out wheras he shud have broken just the middle part of it not the one attached to the surrounding walls. Its just like a ceramic jaali in the start of ur exhaust assembly which gets clogged due to contaminated and leaded petrol in Pakistan...takes about 35000km of running for that to happen. The breakage of catalytic converter increased the car exhaust noise which was fixed 99% when i got a jdm 95 civic dholki fixed after the exhaust and before the tail pipe...means under part of the car. then onwards the problem was totally solved. Nowadays i have heard that there are new ways to CLEAN OUT the catalytic converter too without breaking it but i aint sure wat thsoe methods are...although wat comes to my mind is simply to put some bathroom acid through that jaali n it will clear the clogging.
3) Third possibility is ur spark plugs....the 1kr-fe engine in this car is a very very advanced engine.....the stock spark plugs that come with it are already irridium one so no need for any better ones like 4 tip etc but wat people dont know is....if the stock GAP on that plug is changed even 1mm then that plug is useless....whenever u want to get the car tuned...always always always get the spark plugs cleaned in front of ur eyes... DO NOT CHANGE THAT GAP...local mechanics are in habit of decreasing it which does work on other older cars. Preferably get new plugs....its stock plug in those days was something like 1000rs for one. a set costed 3k.
InshAllah u will feel a positive difference after uve done all these things. Ur kharcha wont be more that 20k total to get everything in NEW shape.
Rest assured its a guarantee that after this much service done u will feel a difference of at least 30 to 40 percent better performance compared to now.