DEAR TOM AND RAY: Since buying a Prius, I have become overly interested in gas mileage. The dashboard tells me I am regularly getting more than 50 mpg. But when I try to measure mileage the old-fashioned way (actually recording the amount of gas I put in the car and dividing by the number of miles I’ve driven), I come up with a figure about 3-4 mpg lower than what the dashboard claims. So, does the dashboard lie? What about these real-time mileage readouts? Are they any use? Can I trust my Prius? Thanks.
- Pat
RAY: Well, of course you’ve become obsessed with gas mileage after buying a Prius, Pat. You’ve got Prius Syndrome.
TOM: Symptoms include focusing on your instant fuel-economy reading on the dashboard when you should be watching the road, and feathering the gas pedal when starting off from a traffic light, trying to keep the car in electric mode as long as possible, while ignoring the irate drivers behind you who want to know why it’s taking you 25 seconds to get to 15 mph.
RAY: The dashboard readouts actually are pretty accurate, Pat. Better than what you can do yourself.
TOM: One of the auto testers from Consumer Reports told us that, while they don’t rely on them for published results, they’ve found that the dashboard mileage readings from most manufacturers were accurate to within 1 mile per gallon. Not all of them are that good - and some are off by quite a bit - but most of them are right on the money.
RAY: The better ones work by splicing a fuel-flow meter into the fuel line, which measures precisely how much fuel is actually going into the cylinders. So if the speedometer is accurate (which is not always the case), you can get a very accurate reading that way.
TOM: And it turns out that’s much more accurate than the do-it-yourself method. That’s because in reality, it’s very difficult (unless you’re Consumer Reports, with beakers and syringes) to fill the tank to the exact same place each time you fill up.
RAY: How do you know your tank is “full”? When the pump clicks off. Or when gas spills all over your LuckyBrand jeans. That’s a very inaccurate estimate, in reality. And the margin of error only increases when you have a small gas tank, like you have in the Prius.
TOM: So I’d trust the Prius’ computer, Pat. And whatever you’re getting, remember that it’s plenty, compared with what the rest of us jamokes get … unless you hit a tree while watching the mileage readout on the dashboard. Then your mileage will drop significantly! So please drive safely.
DEAR TOM AND RAY: I bought a 2006 Pontiac GTO about a year ago. After a considerable search, I found one with an aftermarket sunroof so the sun’s rays could help heal the psoriasis problem on top of my head. When it rains, or when I go to the carwash, my car fills with water. It sounds like there is water sloshing around inside the passenger door. During heavy rain or while driving at high speeds on a wet freeway, the rear passenger floor gets absolutely soaked. The water then moves forward to “dampen” the front passenger floor. I know what you’re going to say: “Replace/ repair the sunroof first.” OK, I will. But just in case, what else could it be? Thanks in advance for your help.
- “Bathing Bill”
RAY: We can give you a couple of ideas, Bill.
TOM: That water that sounds like it’s sloshing around in the passenger door? It could be sloshing around in your passenger door!
RAY: Water can get into the doors when it rains. That’s why doors have drain holes at the bottom - so the water can get out. But if your drain holes are plugged up with leaves, dirt or dead insect carcasses, water could be accumulating in there. And if a few inches of water builds up in the door, it easily could spill over onto the floor of the passenger compartment.
TOM: So have someone check the drain holes in your doors for you.
RAY: If the door drains are clear and there’s no water in there, then you might check the sunroof’s channels.
TOM: All of mine are set to HBO.
RAY: The water channels in your roof are supposed to drain away the water that gets in through the edges of the sunroof’s opening.
TOM: There’s no way to completely seal up the sunroof, since it’s essentially a hole in your roof (although factory sunroofs tend to be better than aftermarket ones). So all sunroofs have channels that drain out whatever water does get in there.
RAY: There’s one channel on each side of the car, and they run along the edge of the roof, and then down through the roof pillars and onto the ground. Your channels could be all plugged up.
TOM: The water that sounds like it’s sloshing around in the passenger door could be sloshing around in the A pillar, which is the roof pillar that runs along the edge of your windshield.
RAY: And, like water in the door, if water builds up in your sunroof channels and has nowhere else to go, it looks for the easiest path to the ground - which happens to be through your rear carpet.
TOM: Those channels can be checked and blown out with compressed air.
RAY: If that’s not it, another thing to look at are the seals around the doors. If the weatherstripping is damaged or dried out and cracked, a surprising quantity of water can get into the car through the edges of the doors - and that also could end up on the floor in back.
TOM: The same is true for bad molding around the front or rear windscreen.
RAY: And if it’s none of those things, and you run out of ideas, think about just opening the sunroof when it rains. Maybe getting all that water right on your head can at least momentarily soothe that dry-skin issue?
TOM: Don’t be offended by my brother, Bill. He just tries to look at every GTO as half-full (of water) instead of half-empty. I hope you find the source of the leak!
Click & Clack (Tom and Ray Magliozzi) dispense advice about cars in Car Talk every Saturday. Email them by visiting cartalk.com
HomeStyle, Pages 39 on 04/12/2014