Do you mean miles per BTU or miles per pound, or miles per volume of gas tank.
Miles per BTU suggests that natural gas doesn’t convert to mechanical energy as efficiently as petroleum, which implies to me that a natural gas engine must have lower compression ratio than natural gas. I don’t think that is true.
I think miles per BTU. But obviously the car uses the same engine for both fuels.
In any event- the car gets 20% better MPG on gas than it does in cng. When both are measured in gallons. That should be something that carries into a BTU measurement, since we are comparing two combustible fuels.
When it runs on CNG, I find it’s noticeably less responsive when pressing the floor pedal. ie..has to go down closer to the floor when on on cng than gas.