If you want CNG with AMT, you have a limited choice. Tata Tiago is a better option between the two. While Tata Motors has not updated the product in some time now, there are very few CNG AMTs in the market currently. 

If that is your requirement, then the Tata Tiago makes more sense. It offers better cabin space, and it has a usable boot despite the CNG cylinders taking up some luggage room.

The other option you can consider is the Nissan Magnite. It is easier on the pocket and has been updated recently. Maginte too comes with the CNG and AMT combination. 

The Tata Tiago iCNG AMT in XTA trim is the best CNG car in your budget. There is not much of a performance drop when switching from petrol to CNG mode, and Tata's clever use of two CNG cylinders also means you get some usable boot space too. You can read our full review of the Tata Tiago iCNG. 

If you are finding more value in the Maruti Wagon R, you should go ahead with this, as in the end, that is what matters to you. However, do bear in mind that the boot of the Wagon R is completely filled with the large single CNG tank and the spare tyre, thus leaving practically no space for even small bags. 

The Tata Tiago CNG, on the other hand, has the spare tyre underneath the car and comes with a dual CNG cylinder setup which sits below the boot floor, thus leaving you with a decent amount of luggage space.        

The 1.2-litre K-Series engine in the Maruti Wagon R makes it feel noticeably peppier and relatively more effortless than the 1.0-litre petrol version. Also, being a four-cylinder unit, it is a bit smoother and more refined as well. 

Yes, the 1.0 will be a bit more efficient than the 1.2; however, if you like to drive and if it falls within your budget, quite simply go for the 1.2. The Tata Tiago has its strengths, like a good sound system, and better driving manners than a Wagon R; however, in terms of space, user-friendliness, smoothness and overall sales and support, the Maruti car is the one to go for.

CNGs cars and SUVs have come a long way in the past few years. 

Carmakers like Tata, Hyundai and Maruti Suzuki all offer models with underbody CNG tanks, which means you don't have to compromise on boot space. 

As for CNG automatics, there are currently three models for sale in India in three different body styles. 

The Tata Tiago CNG AMT (Rs 7.23 lakh-8.07 lakh, ex-showroom) is a hatchback, the Tata Tigor CNG AMT (Rs 8.19 lakh-8.74 lakh, ex-showroom) is a compact sedan, while the Nissan Magnite CNG AMT (Rs 6.89 lakh-9.70 lakh, ex-showroom) is a compact SUV.

Given your requirement for a car for road trips, we recommend looking at the more powerful options within your budget, rather than the more feature-packed ones.

The Maruti Suzuki Ignis is a good fit for your needs. It has a nice 1.2-litre engine, and Maruti's easy-to-access service network also counts for a lot when venturing out of city limits.

Citroen C3 is the best-to-drive car within your budget, but the service network is small.

Tata Tiago and Tata Punch are also options to consider. They feel tough, and this adds greatly to confidence on a road trip.