In a follow-up to Toyota's first factory-developed petrol-CNG bi-fuel version of the popular Corolla Altis C-segment saloon with manual transmission in early 2009, the automatic variant debuted later in the year wearing a price tag of B834,000 (US$ 25,300) - a premium of B50,000 over the petrol-only 1.6E.

Toyota Motor Thailand claims this car is the first and only mass-market passenger developed in Thailand, with the assistance of its Japanese counterpart, to have passed the EU's crash test assessments.

The only visible change outside is the addition of the CNG badge at the back, while under the bonnet you can see the gas lines and injector wiring neatly installed on top of the valvetrain cover.

The neatly installed 75-litre-equivalent steel tank in the boot capable of holding up to 15kg of CNG is hidden with a bespoke felt-lined cover that also opens for access to the spare wheel - one of the bug found in most other CNG retro-fitted cars.

The refuelling inlet is located right next to the usual petrol cap and the bi-fuel control switch is located at the bottom of the dash next to where the power mirror switch is.

The things you don't really see are the essential modifications like the revised suspension with damping/rebound tweaks, stainless steel gas lines under the body parallel to the standard petrol lines and integration of gas mode in the ECU (not a piggy back type as in after-market installations).

And the most expensive changes are within the engine itself with strengthened valves, valve seats and cylinder head to withstand the extra heat typical of running on CNG. Running cost comes under B1/km.

The one key reason not to buy this car is the lack of airbags and this is not an omission to cut cost, but the E grade Corolla never has this safety device in the first place. If you can wait for most part of this year for the dual front airbags, CNG version of the 1.6G is expected. Otherwise, this Corolla 1.6E CNG is the only mass-market saloon that is thoroughly developed to be petrol-CNG bi-fuel without inherit handling and drivability trade-offs found in all the retro-fitted CNG counterparts like the Chevrolet Optra, Mitsubishi Lancer and Proton Persona.

At B50,000 over the petrol-only 1.6E, the extra cost should pay for itself within only 23,000km of exclusive CNG use according to the maker's calculations.

The bottom line is that this car, potentially, has the lowest running costs among the mass market saloons in this country and with its minimal reduction in drivability compared to petrol-only variant serves up reasons compelling enough to be a good buy.