what is the recommended driving speed for a petrol car with manual transmission to have maximum fuel efficiency?
in India AFAIK it is 60 Kmph
It's hard to say as it depends on various conditions like the gearing of your car but on a perfectly flat road, I'd say put the car into the highest gear and take your foot off the throttle. The speed the car settles down to is just about your most efficient cruising speed: Highest gear, no throttle.
There is no 'per-country' recommended driving speed for fuel efficiency.
It depends on the shape of your car, your engine size and type, the type of fuel you use, the quality of the roads etc.
In my car for example, in top gear I can get 37 mpg at 60mph and it has a gentle drop up to about 75mph, but the efficiency drops off rapidly over 75mph (35mpg at 75 down to 22mpg at 90mph, and dropping further as I get faster) so I tend to use 70 as my general target despite getting slightly better efficiency at a lower speed.