If you want efficiency, the Kia Sonet diesel is the one to go for. Beyond the cost advantage of running a diesel, the Sonet diesel is actually nicer to drive than the Maruti-Suzuki Fronx petrol.
The Sonet 1.5 diesel offers better performance and works well with the 6-speed automatic transmission. The Fronx is, of course, cheaper, but if you have the budget and want a car with low running costs and want an alternative to petrol, then the Sonet diesel is for you.
