(MONEY Magazine) – Q The latest batch of convertibles have retractable steel roofs instead of cloth tops. Seems like you get the best of both worlds--do you?--Bryan Mealer, New York City

A It's a trade-off, and one you may want to make if you live in a cold climate. When the roof's up, you have a steel canopy over your head and a large glass rear window. Put the top down, and it's out of sight. This makes a convertible a more realistic four-season car, but there are two drawbacks. All those motors, panels and pistons can add hundreds of pounds, hurting performance and shaving a few miles a gallon off your fuel economy. And the roof has to go somewhere. It folds into the trunk, so you lose most of your cargo space. However, those are trade-offs some car makers are unwilling to make. Both BMW (in its new G series convertible) and Jaguar (in its new XK drop-top) decided cloth was the way to go.

Q Why is it that diesel fuel can cost more than regular gasoline on one day and less on another? --Pam Carter, Boise, Idaho

A Diesel's a more versatile fuel than gas, so it's used in machines other than cars. What does that have to do with the price at the pump? Well, in the fall, for example, more farm equipment is being used. The more use there is, the more fuel is needed and voilà: Econ 101. Gasoline isn't affected by this kind of fluctuation. Diesel is also used as a heating-oil substitute, so its price may go up again in the winter.