General

A gasoline gallon equivalent, or the amount of CNG it takes to equal the energy content in a gallon of gasoline. Gasoline contains approximately 115,000 BTUs per gallon. 5.66 lbs. of natural gas, as established by the National Council on Weights and Measures, contains the same amount of energy and is sold in units called GGE.
Note: Because of the U.S. Federal Renewable Fuels Standard, virtually all gasoline sold today contains 10% ethanol (alcohol) and therefore has a reduced energy content (approximately 112,000 BTU per gallon).

Virtually all gasoline-powered automobile engines sold today are Otto cycle engines which use spark plugs as the means of igniting the air fuel mixture in the combustion chamber. Diesel cycle engines are used in cars and trucks as well. These engines do not have spark plugs and function by compressing the air-fuel mixture until autoignition occurs. Diesel engines have very high compression ratios and are more efficient than Otto cycle engines. They cost more to produce and their emissions are higher and more difficult to control. Diesels are ideal for high-torque applications like trucks and large pieces of powered equipment.

This is the ratio of cylinder volume with the piston down to volume with it all the way up. Higher ratios are desirable to increase the amount of piston travel and hence work/power the engine achieves with a given amount of fuel. Spark ignited engines are normally limited in the CR they tolerate because a gasoline/air mixture has the propensity to self-ignite before the spark is applied causing harmful knock. Diesel engines function by compressing the air fuel mixture until it auto-ignites, so they naturally have higher compression ratios. Octane is a measure of a fuel's resistance to combustion and hence, high-octane fuels allow the use of higher CR engines—improving power and efficiency.

In most industry discussions, a bi-fuel engine uses one or the other of two fuels, say gasoline or diesel, at a time. A dual-fuel engine uses two fuels at the same time, say diesel and CNG. Occasionally, the dual-fuel and bi-fuel distinctions are reversed in regulatory language. Natural gas is too resistant to combustion to use it alone in a diesel cycle (compression ignition) engine. Compression ignition engines using natural gas will normally use some amount of diesel fuel as well—this is a "dual fuel "engine.

Using CNG as a fuel in an engine which also uses gasoline does not change the basic thermal efficiency of the engine. Compression ratios of spark ignited engines can be increased in engines that only use natural gas because CNG fuel has a very high octane rating of approximately 130.

Generally only heavy-duty diesel engines can be converted to run in a dual-fuel (diesel & natural gas) mode. Components are too large and costs are too high to make this practical for light duty engines.

You should do a careful cost-benefit analysis before deciding on any alternative fuel. Generally, electric is very efficient and will cost less to operate IF your use case is many short trips (< 25mi / 40 km) and you have access to charging (consider your range requirements carefully). A correct analysis will amortize the cost of the battery as part of the fuel/operating cost and correctly account for battery replacement cost at the end-of-use or as part of residual value.

You may wait a very long time. Hydrogen does not exist as a free element. Most hydrogen comes from reforming natural gas in an energy intensive process. It also can be made by electrolysis from water, but this requires input of greater amounts of electrical energy than it yields in usable hydrogen energy. Once formed, hydrogen is less energy dense than CNG and requires even more expensive / larger fuel storage tanks for storage.

Each state determines its own HOV lane access regulations. Please check your state requirements.