Best not to rely on reserve fuel when gauge reads empty

Dear Car Talk:

Please confirm whether this rumor I've heard is true: When the gas gauge is on "E," there are still a few gallons in the tank. I've heard that over the years and cannot find any confirmation. – Diane

RAY: Yes, in my experience, it's true for most cars. Lots of modern cars have "miles to empty" digital readouts now, and those seem to have less in reserve and are pretty accurate. But cars with traditional fuel gauges have, on average, a gallon or two left in the tank once the gauge reads empty. It's what the manufacturers call "moron insurance."

Gauges are designed that way so that if you mess up, and forget to get gas, and suddenly notice the tank is on empty, you’ll still have enough fuel to make it to a nearby gas station. The system was vehemently opposed by the Tow Truck Driver’s Association, who felt it was severely cutting into their 401(k)s. Carmakers rarely talk about the gasoline reserve, with good reason; they want you to forget about it.

It’s like that emergency hundred dollar bill you stuffed into a hidden pocket in your handbag. If you were always aware of it, it’d probably be a pair of shoes by now. Similarly, it’s better not to count on any reserve fuel, and be blessedly relieved that it’s there when you need it. But remember, “a gallon or two” is just our estimate. If you want to know exactly how much fuel remains in your own tank after the gauge reads empty, there’s only one way to find out.

Download the latest “Car Talk” podcast so you have something to listen to, put two gallons of gas in a gas can, and start driving. When the gauge hits “E,” reset the trip odometer to zero. Then check it again when you coast over to the side of the road. Divide that number by your miles per gallon, and you’ll know how much of a reserve you have. And then forget all about it, and fill up every time you get down to a quarter tank.

Car rolling on steep incline stumps mechanic

Dear Car Talk:

I have a 1991 Volvo 240 with 220,000 miles. I had the original clutch replaced at 174,000. When I park on a steep incline, the car rolls. The parking brake never works, but I should be able to park it in gear to prevent it from rolling, right? My mechanic is mystified, and says nothing is wrong with the transmission.

Given that I have no plans to move to San Francisco, it's a minor issue. But I'd like to know what is causing it and get it fixed, if possible. Thanks. – Bill

RAY: This is your most serious problem, Bill?

When you park your car in gear to keep it from rolling, you need two things: You need good engine compression, to prevent the pistons from moving. And you need a good clutch, to keep the engine solidly connected to the wheels. If either the clutch or the compression is weak, the weight of the car (and this is a heavy car) combined with the slope of the hill can overcome the holding power of the engine.

Let’s assume, for the moment, that your clutch is still good since it was replaced 45,000 miles ago. That leaves engine compression. And with 220,000 miles on this old beast, I’d say your compression is the prime suspect. What keeps the car still is that it’s very hard for the pistons to compress the air in the cylinders. If they can’t compress the air, they can’t move, and the car stays still. But if your piston rings are old and worn out and leaky, and can’t hold pressure, it’s a lot easier for the pistons to move. And that’s when you look out the window and see your Volvo rolling down to the 7-Eleven for a Slurpee.

So have your mechanic test your compression. That’s easy to do. And if your engine compression is weak, then you have a few choices. You can rebuild the engine, which I would be hesitant to recommend due to the age and mileage of the car. You can avoid parking on hills. Or avoid parking on hills steep enough to allow the rolling. You can toss a couple of six-by-six blocks of wood in the car and chock your wheels when you have to park on a hill. Or you can try fixing the parking brake.

And if you manage to make it work, you may have the only Volvo 240 in captivity with a working parking brake. But start by getting a compression test. If the compression is good, test the clutch. It’s one of those. My money’s on compression.