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Shantunu, i am not saying that wagonr/santro cannot attain more than 25+ etc. in first gear.. thats not the issue... what i wanted to emphasize that Indica reaches that speed at a lower engine RPM...
(correct me if i am wrong, as i am not too technical about car mechanics..)
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Ok, the deal is that since a diesel engine usually produces it's peak power and torque at very low rpms, it's redlined at a much lower rpm than a petrol engine.
Now the Wagon R has to revv it's engine to 6800rpm in 1st and 2nd to hit those speeds i mentioned. Since the Wagon has a relatively higher revving engine, then gear ratios need to be short, to compensate for the lack of power and torque at low rpms.
Now, the indica diesel "i think" redlines at 4500 rpm. Which means that max power and torque are produced below that engine speed.
If i were to use the same ratios as that of the Wagon R in your Indica, you would have spent all your time concentrating on the gear shifts bcoz, the moment you redline in 1st and shift into 2nd, your engine speed would have already reached close to it's redline.
Ok, i'll make it simpler, now imagine i'm driving a Wagon R with a rev limit of 4500 rpm. So, the max speed i'd be able to hit would be :
30.1 kmph in 1st
54.38kmph in 2nd
80.4kmph in 3rd
112kmph in 4th
136kmph in 5th
Though these gear ratios would make the Wagon R peppy, whats actually happening is that your cruising rpm will suffer a lot, you'd be doing high rpms in 5th gear at 80-100kmph which will hamper your highway mileage, you won't be able to stay in one gear for long leading to a very irritating drive and i'f you've noticed, the top speed of the car has been reduced to 136kmph, where as the present Wagon R manages 145-150kmph.
In such cases you have 2 choices, you either recalculate your gear and final drive ratios or, (if you want a peppy car) add another 2 gears such that your car manages it's top speed of 145kmph without sacrificing the peppyness of the car.
If your Indica was to have 6-7 gears, it would have revved much faster to it's redline but, that would have reduced your max speed in each gear and your FE would have reduced.
So, what TATA has done is that they've calculated the ratios such that even though the car doesn't feel all that peppy, it'll still hit it's top speed within those 5 gears and due to the tall nature of the ratios, you can cruise at 100kmph in 5th at a low engine rpm which not only makes the car very fuel efficient but, also increases engine life.
Shan2nu