Q. Dear Tom and Ray:

I have a 1989 Ford Ranger with a 2.3-liter engine and a five-speed manual transmission. I used to get 23 to 29 miles per gallon. In the space of less than a month, however, the mileage has dropped to between 16 and 18 mpg. My mechanic has checked all of the basics--timing, plugs, filters, electronics--and everything appears to be fine. Any ideas on where else to look?

-- Steve

A. TOM: Check the thermostat. Ford thermostats are notorious for failing, and that would explain a drop in mileage.

RAY: If the thermostat isn't closing, the engine will never get up to operating temperature. And if it's running cool, it'll be running inefficiently, hurting mileage.

TOM: I don't know why Ford thermostats keep failing, but it's almost a joke at the shop. Some guy comes in with a Ford and he wants new tires. We write up the work order as "new tires and a thermostat." A woman brings a Ford in for an oil change. We write down "oil change and a thermostat."

RAY: Some guy in a Ford stopped to ask directions the other day, and we changed his thermostat, too.

Dear Tom and Ray:

Please help me out. The tires on my 1994 Buick are original. They still have good tread, as we have only 12,000 miles on the car. My wife insists that the tires are getting old and need to be replaced. I say they're still perfectly good. Am I right, or do I have to wash the dinner dishes for a month?

-- Walter

TOM: Well, you should wash the dinner dishes for half a month, Walter. That has nothing to do with the tire question; I just think in the interest of fairness, you should help out a little more around the house. Don't you? Then your wife might not be so interested in the tire guy.

RAY: You happen to be right about the tires, Walter. Tires don't spoil after a set amount of time like, say, Roquefort cheese. They're generally good until either the tread wears down, the tread or the sidewall gets damaged, or the sidewalls get severely dry and cracked.

TOM: Sidewall cracking is the most likely cause for concern, especially if you live in a hot climate. But even that shouldn't be an issue after five years.

RAY: Rather than ask a tire dealer to check for you (due to the obvious conflict of interest), ask a regular mechanic you trust to look at the tires. And if he says they look okay, just keep driving.

TOM: By the way, if you tend to keep your tires for eight to 10 years (if you drive only a few thousand miles a year) and you live in a hot climate, you can help delay cracking by treating the tires with Armor All or some similar protectant with UV blocker. Just be sure to coat both sides of each tire, since cracks on either sidewall can put a tire out of commission.

Dear Tom and Ray:

The oil in my 1988 Oldsmobile is so clear that I can't read it on the dipstick. I use 5W-30. What can I do to darken it?

-- James

TOM: Don't change the oil as often.

RAY: Seriously, if your oil is remaining clear, nothing is wrong. It just means it's not being given enough time to do its job. When the oil circulates, it collects dirt, soot, acids, stray pairs of pantyhose and other "contaminants" in the engine.

TOM: When you change the oil, you flush that stuff out. But if you change it too often, you waste both money and oil (and time, if you're doing it yourself).

RAY: If you're changing the oil more often than every 5,000 miles, you need to cut back, for financial and environmental reasons. And the oil will darken and be easier to see.

Got a question about cars? Write to Click and Clack in care of this newspaper.

(C) 1999 by Tom and Ray Magliozzi and Doug Berman