Consumer spending outside of cars and gasoline fell for a second straight month in May, according to SpendingPulse, the retail data service of MasterCard Advisors, an arm of MasterCard Worldwide (NYSE:MA – News).

Retail sales less autos rose 0.6 percent last month on a seasonally adjusted basis, compared with a 0.1 percent uptick in April. “Gasoline continued to bolster retail sales growth … but it’s taking away spending on other discretionary items,” said Kamalesh Rao, director of economic research at MasterCard Advisors. U.S. retail gasoline prices topped $4 a gallon on average for the first time last week, up from $3.67 a month ago and $3.10 a year ago, according to the American Automobile Association. Retail sales excluding cars and gasoline fell 0.5 percent in May, following a 0.7 percent drop in April. Moreover, “core” consumer spending — which excludes autos, gasoline and building materials — posted its biggest drop since February, SpendingPulse said. Core sales tend to be less volatile on a monthly basis, more indicative of overall spending trends.

Core consumer spending fell for a fourth straight month, despite expectations of a flurry of purchases from government tax rebate checks. SpendingPulse’s seasonally adjusted core retail gauge was down 0.5 percent in May, steeper than the 0.2 percent decline in April. It was down 1.0 percent in February. Meanwhile, the U.S. government will release its own retail sales survey at 8:30 a.m. EDT on Thursday.

Economists forecast the government’s overall May retail figure would show a 0.5 percent rise, rebounding from a 0.2 percent fall in April. They predicted its ex-auto reading would show a 0.7 percent increase compared with a 0.5 percent rise in April.

The SpendingPulse data are derived from the aggregate sales in the MasterCard U.S. payment network, coupled with estimates on all other payment methods including cash and check.

Of course, large categories where e-commerce has hardly made a dent—such as autos, gasoline and groceries—skew the results.

The DOC data illuminates a seasonal trend: Online sales penetration is greatest in Q4. A likely reason for this is that the benefits of e-commerce—convenience, broad selection, cost savings, free shipping and avoiding crowded malls—shine brightly during the holiday shopping season.

Forrester Research used its own proprietary formula in May to estimate that e-commerce will represent 7% of retail industry sales in 2008.

Even some of the categories with online sales penetration rates that exceed the industry’s 7% rate still have strong growth potential. Forrester estimated that 20% of consumer electronics sales will take place online this year. Other Forrester data showed that online sales in this large category could grow to 29% of total sales by 2012.

Moreover, some of the least-penetrated categories such as auto/auto parts (2% penetration) and food, beverage and grocery (1% penetration) nevertheless generate respectable online sales.

Take groceries, for example. Many of the early problems that plagued online grocers are being addressed today through innovative business models. In this light, the fact that most groceries are still sold offline could be viewed as an opportunity for online food merchants who can figure out how to lure customers from physical stores.

But a FreshDirect senior vice president said the upgrade would not affect the equipment that has led to occasional complaints about the company — a smaller motor that runs refrigeration equipment to keep the food fresh. The new equipment will shut off only the engine that powers the drive train. The two operate separately.

The state attorney general, Andrew M. Cuomo, announced Friday that his office and FreshDirect, based in Long Island City, Queens, had reached an agreement on installing the shutoff equipment after an investigation into consumer complaints that FreshDirect trucks were violating anti-idling laws.

A statement from Mr. Cuomo said the investigation documented at least 30 cases of illegal idling by FreshDirect trucks. Under state law, trucks and buses cannot idle for more than five minutes at a time. New York City limits idling time to three minutes, and in areas near schools, it is no more than 60 seconds.

FreshDirect, which averages 7,000 deliveries a day, has agreed to pay a $50,000 penalty for violating state and city anti-idling laws, the statement said. It said the penalty had been $120,000, but $70,000 had been suspended contingent on the company’s compliance.

Mr. Cuomo said that besides affecting public health and the environment, idling wastes fuel: an average of 30,000 gallons of gasoline and 20,000 gallons of diesel fuel in the city every day.

Jim Moore, FreshDirect’s senior vice president for business affairs, said the company has had experience with engine-control technology. It has the equipment on 20 to 25 trucks, the newest in its fleet. The engine turns off if the vehicle remains in park for more than three minutes.

The company promised that any new trucks would come with the equipment.

Mr. Moore said the company had received complaints about idling since its trucks hit the streets in 2002. He said the company had looked into those complaints and concluded that they stemmed from noise made by the refrigeration engines. Those are not covered by the agreement with Mr. Cuomo.