I looked for one car in the gov fuel ratings. I found a 2003 Honda CIVIC that came both as a gas burner and CNG burner.

Gov (EPA) ratings

2003 Honda CIVIC 1.7L GAS
31-34-39 mpg

2003 Honda CIVIC 1.7L CNG
26-28-31 mpg

A bit less I'd say................

Of course what is being forgotten, is the ability to find CNG while on a road trip. Which to me is about making as much sense as my 12 year old dog who is currently chasing her tail. But that is just me.

... Better your dog should chase it's tail, than the neighbor's car ...

or your dog huffing CNG

I can see derf huffing the dogs tail while John's dog chases them down the road in search of CNG trying to get to Oceans.

Hey I stick to high quality paints (no offense to John's dog's tail)

Dual
 

The conversions kits allow for dual fuel use so when you are going for a long trip you can just switch to gas. otherwise you can use a home compressor station and never have to go to a gas station.

I need to do more research on the MPG rating though. The prices you see at gas stations are for gge's gallong gas equivalents which should be the same amount of natural gas energy for an equivalent gallong of gas energy. Therefore the difference in mpg's should be taken into account unless cng is just less efficient for some reason.

CNG is less efficient and taking that into account gives one the cost per mile to use. Lower price, lower mileage, cost of kit, cost of install, cost of maintenance. All of this on an old car that may die at any time. Kinda reminds me of the guy that buys $500 dollar wheels for a $900 dollar car.

CNG is great but you need a motor that will optimize it's use meaning the CNG is around 120 octane so make your compression ratio around 11 and use a cam that will make peak torque at 3-5k also you need an exhaust at 2.25 in. After all this you'll find that it does not cost much different to fill the tank than gasoline but it should make more power.