MANCHESTER, N.H.

MOTORISTS who modify an old diesel vehicle to run on straight vegetable oil make use of a greasy byproduct of the restaurant industry, reduce demand for petro-fuels and don’t use up materials needed to make a new car, but they get little respect.

The Internal Revenue Service gives tax credits to buyers of brand-new hybrid and lean-burning vehicles, but gear-heads who modify their own vehicles to run on straight vegetable oil get no tax credit.

The reason is partly that tax credits are designed to maximize the introduction of new energy-efficient vehicles into the market, U.S. House Committee on Ways and Means spokesman Matthew Beck said.

Secondhand hybrids and lean-burning vehicles, which increase efficiency with a higher air-fuel ratio, get no tax credit because the used vehicles have already made it into circulation and the tax credit has achieved its purpose: getting more energy-efficient vehicles on the road.

But vehicles running on vegetable oil bring a value-added modification to the secondary market.

The reliance on petro-fuels is correlated with a raft of hidden expenses, such as acid rain, smog, other air pollution and its health costs, geopolitical conflict, and, possibly, catastrophic results from global warming. Taxpayers who pay the bill for these hidden costs should be allowed to benefit if they contribute to reducing demand for petro-fuels. But the prospects are not promising.

No legislation has come forward pertaining to veg-oil vehicles, and, meanwhile, legislators are back-stepping from providing tax incentives for hybrids and lean-burning vehicles.

The Energy Policy Act of 2005 created a tax credit for some hybrids, but those credits are already beginning to run out. For instance, buyers of the Toyota Prius, the darling of the automotive industry, will get no tax credit if their purchases came after last Oct. 1.

The Ways and Means Committee created a new energy bill to take away $13 billion in subsidies to oil and gas companies and provide credits to the buyers of next generation vehicles, such as hybrid plug-ins, but in December the Senate killed the bill 59 to 40.

This means no new alternative-energy vehicle credits next year, and small-scale biodiesel manufacturers will no longer get a tax credit of 10 cents per gallon. (Biodiesel is produced by mixing methanol, vegetable oil and a catalyst and requires no modifications to a standard diesel engine.)

Drivers who modify their used vehicles with a heating system to burn straight vegetable oil (the heaters lower the viscosity of vegetable oil to a level similar to petro-fuel and lets it pass through the fuel pump and injectors) seem further away from getting their due.

Alternative-energy contributors have long faced powerful opposition.

Rudolf Diesel did when he introduced an engine that ran on peanut oil at the Paris Exhibition in 1900. He disappeared 13 years later over the deck of the steamship Dresden on a trip to England from Germany. Near bankruptcy, he may have committed suicide. Supporters claimed his death was either engineered by U.S. officials who opposed his selling of the diesel engine to the French and British navies or oil executives concerned that peanut oil could replace petroleum.

To this day nobody knows for sure. The diesel engine, meanwhile, gave motorists a way to ride on recycled veg-oil while the petroleum-based automotive industry remained in fuel-efficiency stasis for nearly a hundred years: Consider that a 1925 Ford Model-T got 25 miles a gallon and a 2005 Ford Taurus claims 25 mpg on the highway.

Jim Kozubek lives in Manchester and writes for the New Hampshire Union Leader and other newspapers. He drives a 1989 VW Jetta diesel that runs on straight vegetable oil.