I never said Cultus had a problem. All I said was that if you are needing spare parts for a brand new car, that means the car is faulty/sub-standard.
Now, let me come to the technical specs where Santro outperforms Cultus. Cultus is based on the standard internal-combustion carburator based engine. This engine is purely mechanical in nature hence has no idea on how much fuel to pump into the carburator at what time. Hence, what happens is that the same amount of fuel is pumped at all times into the carburator no matter what the status of the car. What this effectively does is that in case you are striving for acceleration, less than required fuel is present and this decreases acceleration. Same happens if you are going up uphill. If you are standing still with the engine on (for example on a traffic light), then more than the required fuel is provided which decreases efficiency.
Santro has a multipoint fuel injection (MPFI) engine. It has sensors all over the engine to detect what sort of environment the car is running in. Whether it needs more fuel or less fuel. The fuel is injected directly into the cylinders accordingly without going through a carburator (there is no carburator is a Santro). Hence, the fuel efficiency is increased.
Another area where Santro has an edge over Cultus is that Santro has 4 cylinders whereas Cultus has only 3. 4 cylinders help with a better acceleration.
Now let's talk numbers. Santro generates 55 horsepower at 5700 RPM. Cultus generates 55 horsepower at 5800 RPM. As you can see, there is a difference of 100 RPM between the 2 readings. This means higher engine turning for Cultus which automatically implies higher fuel consumption.
The horsepower that is generated by both cars is the same. I think you would probably know that Santro weighs less than Cultus. Hence the power to weight ratio of Santro is better than Cultus. Which autmatically makes it a faster and more economical car.
With all these benefits, Santro is a cheaper car than the Cultus (atleast it was back when I bought it!).
The only area where I feel Cultus is better than the Santro is that it has more leg space.
Now I would like to re-emphasize that there is nothing WRONG with Cultus. It is just that Santro is a better car.