Brother mehran & cultus are most fuel efficient if u can find CNG.
City vario and carby indus corolla 2E on petrol.
All japanese cars are gas guzzler no matter 660 cc, majority of mira / vitz / passo goes 9-10 km in 1 litre.
City Exis 2001-2 or City Idsi 2003-4 both are good considering your long travelling. Cultus 2008 model can also do good to you.
Go for Daihatsu Mira, a friend of mine have it, hes using it in petrol and its giving awesome fuel consumption, much better than city.
JDM 660cc cars, I would not recommend for frequent highway traveling.
for long travelling , use bigger car , and city is best for u , in 900 k. mira is better in city driving only.