DEAR CAR TALK: I am the proud owner of a 2017 Honda Ridgeline that, unlike my previous 2006 Ridgeline, has all kinds of fancy features including a digital readout that lets me know how many miles per gallon I'm getting.
The problem is, the information isn't accurate.
My wife and I recently went on a short road trip and I was pleased to see that we averaged 30.4 mpg. But when I filled up with gas and divided the number of gallons into the mileage driven, I actually achieved 27.06 mpg or 89% of the digital reading. Ever since, each subsequent actual mpg has been exactly 89% of the digital reading.
I've skimmed though the 500+ pages of the digital owner's manual to see if an adjustment can be made to reflect actual mpg but to no avail.
Is this a problem with other makes and models? Any suggestions other than using a calculator every time I fill up?
-- Ernest
DEAR ERNEST: Well, it's hard to know just how accurate these onboard gas mileage readings are.
The manufacturers insist they're very accurate. They say the same precise instruments that measure your fuel economy are also used to run the engine management system, where fuel and air have to metered very precisely to maintain pollution standards.
On the other hand, you can understand the incentive for overstating the mileage a bit. If you get 30.4 mpg in your pickup truck, not only are you happy with your purchase, but you might tell a friend, who could go out and buy his own Honda Ridgeline, right?
There was a study done by Edmunds that concluded onboard gas mileage readings overstate mileage by about 5% -- with specific cars varying more or less than that. But that was done in 2011, so we don't know if the results still hold.
What we can tell you is that the onboard gas mileage readings are consistent. That means you can use them to tell whether your mileage has increased or decreased. If you got 30.4 mpg one weekend and 32.4 mpg the next weekend, you'd know that something really did change.
And that's what they're best for.
Interestingly, these dashboard readouts have been proved to increase people's mileage. How? When you drive more gently, keep your tires properly inflated, slow down a bit and take those 4x8 sheets of plywood off of your roof, you'll see your mileage go up on your dashboard. And when humans see good results, they tend to repeat whatever got them good results. So these dashboard readouts "train" us to drive more efficiently.
And keep in mind that your homegrown measuring system may not be any better. Filling up the tank and dividing the number of gallons by the number of miles is not perfect either -- unless it's done over many, many tanks full of gas and thousands of miles. You don't know precisely how full your tank is each time the nozzle turns off, nor can you account for differences in weather, road conditions or terrain.
So, if you're very confident in your own measurements, Ernest, you could just reduce the dashboard figure by 10% to get an mpg number you feel you can believe in.
But my suggestion would be to use the dashboard number for comparison. Use it not as an absolute number, but to teach you how to improve your mileage.
DEAR CAR TALK: My wife drives a 2019 VW Tiguan. When I occasionally drive it in the winter, I notice she has both the heater and the AC on.
She claims the owner's manual says to have the AC button on to remove humidity. But I can't figure out why you'd have the AC compressor running at the same time as the heater. Why?
-- Richard
DEAR RICHARD: Like your wife said, it's to remove humidity. You should listen to her. In fact, maybe I'll write to her next time I get a question that stumps me.
The way air conditioning works is by removing humidity from the air. That's the "conditioning" part of air conditioning. It cools the air, too, but removing humidity is a big part of what makes you feel cooler and more comfortable.
Why would you want that in the winter? Let's say there's humidity in the air, which there usually is, and let's say the outside of your windows are cold because it's, what? Winter!
Now, when you heat up the inside of the car, you have cold glass on the outside and warm glass on the inside. That causes condensation or fog to form on the inside of the windows (see also: beer glass). The result? You can't see!
But if you remove the humidity from the air inside the car, there's nothing to condense on the inside of your windows and fog them up. That's why you run the AC. In fact, many cars automatically turn on the AC when you activate your defroster.
In terms of whether you're "wasting" something by running both the AC compressor and the heater at the same time, you are, a little bit, but it's small potatoes.
While the AC compressor does use energy and reduces your mileage a bit, your heat is really free. With an internal combustion engine, your cabin heat is excess heat that's being thrown off by the engine. And it's being produced whether you're sending it to the passenger compartment, or letting it dissipate into the atmosphere.
I think most people would agree that it's better to spend a few cents more on gasoline -- and use your AC compressor when you need it -- than to let your windows fog up and drive into the back of the taco truck.
Ray Magliozzi dispenses advice about cars in Car Talk every Saturday. Email him by visiting:
cartalk@com
HomeStyle on 04/04/2020