Drive it forever

Q. I have a 2007 Nissan Quest that computes mileage on a continuous basis. How accurate is it when it says that I averaged 23 miles per gallon for a tank of gas?

When I cross-check the computer by dividing miles by gallons at fill-up, I generally come up to half to 1 mile per gallon less than the computer. I want to chalk it up to variables that the computer takes into account, and thus the computer is right - or am I suffering from wishful thinking?

A. My experience parallels yours: The economy displayed on the dashboard can be some percentage off from the economy calculated from the trip odometer and the pump delivery.

Sometimes it's high, sometimes it's low. And I find that it's different depending on whether the mileage is highway miles or town miles.

Q. What do you think about the need to perform buffing of alloy wheels, at $25 per wheel, to stop slow leaks from tires?

The tires on my 1998 Honda Accord are about 17 months old and have been driven about 10,000 miles around Boston. The two rear tires developed slow leaks that required reinflating to normal pressure about once a month, so I took the car in to my tire dealer. He advised me that they did free repairs only for a tire problem and, if it was a wheel problem, they charge for buffing the wheel bead-contact area to remove corrosion. I asked about just putting a sealer or lube on the bead, but he indicated that this was not sufficient.

Is buffing a necessary procedure to ensure adequate sealing, or is it a way to get more money from the customer?

A. Both steel and alloy wheels sometimes need to be buffed to fix bead leaks. Corrosion, caused by water and salt, leaves small pits in the metal, which have to be smoothed out and then painted over to prevent corrosion.

Demounting, buffing, painting and remounting for $25 per tire? Sounds about right. I think the dealer has a right to charge for his time.

If he's installing tires anyway, and the wheels have already been stripped of the old tire, it should be less than $25. But not free.

Q. The a/c-heater blower fan won't shut off on my 1997 Mercedes-Benz S 500. It started with the blower fan turning on and off on its own, and now it won't shut off. I have to disconnect the battery to turn it off and, when I reconnect the battery, it turns on again.

Is there a computer module that may have gone bad? I can't even control the fan speed.

A. There's a technical-service bulletin concerning water infiltration into the harness that can keep the blower on. Otherwise you'll have to replace the blower motor and controller as a unit. The Mercedes-Benz dealership can help you with the TSB.

Q. I have been reading Popular Mechanics for some time now, trying to find the best car to replace my 1989 Ford Escort wagon with a six-speed, manual transmission. It is approaching 100,000 miles, and I would like to buy something that gets as good gas mileage as I get with this car - 40 mpg - but I haven't found one.

Why was it possible for the car companies to make cars that actually got 40 mpg in 1989, when they can't seem to do that today?

A. Part of it is the extra weight associated with crumple-zone front ends, rollover protection and the odd half-dozen air bags on board all new cars.

Another part of it is the huge reduction in emissions output required by the EPA in the past 20 years.

And part of it is the fact that the real, inflation-corrected price of gasoline at the pump steadily declined from 1989 until only about two years ago.

Mike Allen is a senior editor for Popular Mechanics magazine. Questions should be sent via e-mail to driveit@nytimes.com.