The price spread between gasoline and compressed natural gas has narrowed, putting vehicle conversions on a downward trend until it is now steady.

Vehicle conversions slowed about 15 percent in the last half of 2015, said Tom Sewell, president of Tulsa Gas Technologies, Inc., which makes all the equipment for the modification.

The price of CNG has held steady at $1.14 per gallon.

Cost of conversion can still save an owner, but it takes longer, 60,000 miles instead of 40,000 for a three-quarter ton pickup, he said.

Back in the 1980s, it was easy to make a conversion by adding a second carburetor. The fuel tank became more complicated in the 1990s as the result of changes in automotive technology, such as replacing carburetors with electronic fuel injection.

It got real complicated about 2000 as a result of rules from the Environmental Protection Agency which now requires that the engine, chassis and model year for each vehicle be certified.

For example, he explained, two certifications are required for pickups and vans each equipped with the same engine.

The rules “frustrate us like crazy,” Sewell said.

Certifications can cost as much as $100,000 with the price per conversion going from about $3,000 in the 1980s to more than $11,000 today, he said. Conversions take about two days.

Currently his company converts GM and Ford pickups and vans and some Chevrolet Impala sedans.

CNG tanks are pressure vessels, and the laws of physics dictate they either be balloon or sausage shaped. For the same range as gasoline, a CNG container is about five times larger.

In addition to converting vehicles, the company also makes dispensers for stations and field calibrators used by state inspectors to ensure there is an accurate measure of the fuel being delivered.

The company’s fueling dispensers have been installed from coast to coast.

A sister company — Blue Energy Fuels — also operates CNG stations in Owasso, Grove, Mannford and two in Tulsa. In April, there will be one in Broken Bow. All but one in Tulsa are unmanned and only accept credit cards for payment.

There are about 75 stations in Oklahoma and another 22 planned, he said, ranging from Guymon to Broken Bow to Frederick to Miami.

It takes five to seven minutes to fill a CNG tank at the station. A commercial dispenser costs about $39,000. A dispenser at a home, at a cost of about $8,000, would take overnight to fill a vehicle at about a gallon, he said.

Sewell said he recently drove to Atlanta and back using only CNG in his pickup, which has a range of about 300 miles.

He has a company purchasing policy of first buying in Tulsa, the surrounding area, the state and then the country.

Building a one dispenser station easily provides jobs for 1,000 people from those putting down the concrete to their office and throughout the supply chain, Sewell said.

Blue Energy buys its fuel on a take-or-pay contract by the year and so do some of his fleet customers.

Towering over one corner of his desk in his crowded office are four large computer screens.

With them he can track the price of gas forward and backward in time, current activity at a station, including one in Houston under contract and other business data.

The company’s expertise is not limited to the U.S. It partners with a company in India that sells dispensers in India, Pakistan, Egypt, Bangladesh and China.

Twitter: @tulsabusiness