Cloth Top Increases Wind Resistance, Fuel Usage Slightly

Article excerpt

Dear Bob: I own a 1996 Lincoln Town Car. I previously owned 1990, 1992 and 1994 model-year Town Cars. The older cars consistently gave me 18 mpg around town and 26 mpg on the highway.

My 1996 model has 8,000 miles on it and gives me 16 mpg locally and 21 mpg on the road. The service department at my local dealership ran extensive tests and could find nothing wrong with the car.

My salesman explained that the cloth convertible top causes the bad-mileage problem because of wind resistance.

What's your opinion?

- L.K., Middletown, Pa.

Dear L.K.: In my book, "How To Get More Miles Per Gallon in the 1990s," I wrote "Would you believe a vinyl top can reduce your highway gas-mileage figure? It's true. A vinyl top adds extra resistance to smooth air flow over the top of the car, and it could cost you 1/2 mile per gallon."

Now, while vinyl isn't cloth, there's no question that your pseudo-convertible top costs you a bit of fuel economy, especially on the highway. But I do mean "a bit" - definitely not the 5 mpg highway difference between your old and new cars.

Around town in particular, the top should be a nonfactor, because you have to get up to about 45 mph before wind resistance and drag become measurable factors.

While I can't tell you what is causing the decrease in your fuel economy, I can tell you that it's not all due to the cloth top, as the salesman implied.

Dear Bob: I would sure appreciate a definition of gross- and net-horsepower ratings.

- J.M., Hanson, Mass.

Dear J.M.: According to the Dictionary of Automotive Engineering, published by the Society of Automotive Engineers, net horsepower is the brake horsepower of a fully equipped engine, while gross horsepower is the "measured power output of an engine operating without power-absorbing ancillaries. . . ."

Dear Bob: I just purchased a 1997 Ram pickup with a 5.9-liter gas engine. I'd like to know if you consider it safe to use a synthetic-blend motor oil after 3,000 miles. …