l am having Maruti Suzuki Alto

Get your engine tuned properly and drive at a constant speed. Try to aviod the use of clutch as much as u can. Get type pressure checked.

replace fuel and air filter

Keep it tuned up, change the air filter when it look dirty, replace the fuel filter every 20K miles. Air your tires two pounds above the recommended pressure. Get rid of unneeded weight (books, trash, extra tools and anything you do not need to carry with you. Drive with the windows when you can to help stop extra drag and do not take off fast and run 55 on the freeway. Good Luck and God bless

Plan ur route carefully. Take the route with the fewest stops & turns & least traffic. Take highways in preference to city streets when possible.

Slow down. The faster u move, the harder ur engine has to work to push through the wind. Speeding can reduce fuel efficiency by up to 33%.

Select a gear ratio that is appropriate to ur engine, transmission, & driving conditions. If u travel on the highway often & do not carry heavy loads, try a numerically lower final drive ratio (known as a ''higher'' gear.) Careful not to go too ''high'' with the ratio, this can cause smaller engines to work too hard to keep up, damaging the engine. Some manufacturers offer optional gearing.

First of all u need to clean the air filter .After that u must clean the spark plug or replace it if it is very old . You must maintain an idle tire pressure of 16 psi .While driving u must drive at steady speed of 55Kmph. Further u must use Xtra premiumor any other good quality petrol. Add also additive in ur petrol which is available in the market . Get ur car proper serviced & inspect the fuel line by any expert mechanic. drive with 4th or 5th gear even at low speads of 35 to 40 kmph. Avoid frequent breaking & acceleration, drive at a constant speed.
l am sure that ur car average will reach till 20 & above.