well, first things first. i DONT study hard for exams 
so i really need ur well wishes to pass. thanks for those.
Moving on to the topic under discussion. i believe we both were talking about the same thing and we both understand how things work but we both were looking at the subject from a different perspective. I was saying that even if u somehow double the torque available in an engine, the car would still do a specific speed (say 60km/h) on a certain RPM (say 3000) UNLESS you change the gearbox.
And you are saying that if u double the torque of an engine, the engine will have the ABILITY to do 60km/h at much lower RPMs (say 1500) and yes you are right. I do believe the engineers look at a lot of factors when deciding the gear ratios and one of them, infact the most important one of the would be gear ratios. So i think we both agree the simply giving a car a larger engine (everything else remaining same) would NOT enable it to do a certain speed in lower RPM then before ALTHOUGH it may now have the ABILITY to do so.
btw, every now and then i hear guys saying 'meri civic dosray gear mai 110 karti hy'. i tell them their civics are awesome but i actually laugh at their understanding. they somehow believe that indicates the power or health of the engine which it does not! its about gear ratios.
Anyways, i have highlighted some of the phrases in ur posts that i dont agree with or i dont understand. i ll go through them one by one.
1) Tire change does not actually effects the fuel economy (if thats what u mean by average) if u increase the width, THAT effects the economy but if u increase the height (profile) of the tire, then the gear ratios are disturbed and ur car wud do more speed then ur speedo is telling u. so the REAL speed may be 105km/h when the speedo is showing 100km/h. im sure u understand this but u know, open forum, we gotta keep things clear for everyone.
2) Understanding the concept of BHP and torque is very difficult. or atleast, thats what i felt and even today i do not have complete understanding of it. what i understood is, torque is how MUCH work an engine can do and BHP is how FAST the engine can do that work. See diesel engines have lot of torque (ability to do work) but very less bhp (ability to reach the maximum output as quick as possible) so they accelerate slow but can climb hills or haul luggage easily. i believe, since we are not talking about acceleration, bhp is not relevant here. infact, engines dont even MAKE bhp. they make torque, and no machine ever in the world has been made to calculate bhp of an engine. the dynos take torque figure and puts them into a formula to get the bhp figure. mentioning bhp here wud mean we are talking about how FAST a car can reach 60km/h in 2nd gear. but we are talking about rpm and speed only.
3) Again, it wud run out of torque, not bhp.
4) I did not understand. how does a clutch provide power? it only transmits it and it also wastes some of it during shifts, by slipping a little bit.
5) thats not correct. im pretty confident. I googled it and found a lot of sources that claim petrol to have a max of 35-40% efficiency and diesel a bit more then that. The ICE (Internal combustion Engine) is not a very efficient form of engine. I believe the rotary configurations would give the best efficiency but still not even close to the figures u quoted.
6) why wud anyone, including me, take any offence out of that post? 
we're all here to 'buy,sell,discuss' remember? 
