Dear Tom and Ray:
Since buying a Prius, I have become overly interested in gas mileage. The dashboard tells me I am regularly getting over 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 good, 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 Lucky Brand 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.
(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk website.)