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#1341: Kitty-Corner Snow Tires

Oct 12, 2013
This week on Car Talk, can Dave put two snow tires on his girlfriend's car, or does he need to buy a full set, as his tire dealer insists? Tom has a two-tire solution that may qualify for next year's Nobel Prize in Bad Advice. Elsewhere, Cathryn's car is going "boom boom" whenever her mechanic isn't around to hear it; Helen's Toyota gained and then lost an extra 100 miles per tank of gas after she moved to Salt Lake City; and, can Keith install an air conditioner in his Geo Metro, and still make it up a hill? Also, on Stump the Chumps, we find out if Tom and Ray got Pam's Volvo to stop smoking. All this and more, this week on Car Talk.

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The Empty Brake Fluid Can: What one question will reveal what's wrong with the car?

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8 Comments

Helen's MPG!

In a previous comment it was stated that a truck would not be effected by aerodynamics. WHAT? NO SOUP FOR YOU! The fundamental concept of aerodynamics would explain this 100%. Unlike liquids and solids, gases are composed of discrete molecules which occupy only a small fraction of the volume filled by the gas. On a molecular level, flow fields are made up of many individual collisions between gas molecules and between gas molecules and solid surfaces In simple terms the more drag a vehicle has the more it would be effected by air density. Refer to my previos post about your hand in water. Imagine if you turned your hand sideways vs upright. Big difference right? Also if you listen to what she said, she noticed the difference before she changed her tires and another difference after. This would rule the tires out as the main culprit. I am sure there are many different factors. Air temp, driving surface, road topography, user changes etc... But I feel the main culprit will be air density.
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Fuel Economy

One of the reviewers said he thinks the change was due to the air density. Unlikely that aerodynamics on a Toyota truck would have much to do with fuel economy. One of the other reviewers nailed it though and you guys totally ignored it. The tires. You neglected to ask if the new ones were larger, heavier, etc. Tires have a large effect on fuel economy for a number of reasons. New tires even of the exact same size and brand are heavier and larger in diameter because they are not worn yet.
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MP gee

U should note that in UT and several surrounding states, the gas is often at 85 Octane (regular). Unknown if this is the case in WI, also a number of western states reformulate gas in the winter (using ethanol and/or refinery adjustmets); UT has refineries. The answers about altitude and tire pressure are also reasonable. Do we all need remedial math? My 2010 Chevy "smart" gusge reads mpg pretty well. Calibrate! Hey, lay off Berkeley, plenty of state-of-the-art car mechanics and alternative fuels folks there.
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opening and staff listing
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Helen's MPG!

As always I lisen to your podcast Monday afternoon. Which is probably why I get nothing done and may be asking Ray for a job soon. However, This week's episode struck me personally. When Helen called in about her MPG gain. I knew right away your answer was incorrect! B-o-o-o-o-o gus! No stump the chump for you! You see, in 1999 I moved from Hilton Head Island South Carolina to Tucson Arizona. Then in 2006 I moved to Eau Claire WI. All with the same vehicle. My prized 1991 Ford Taurus SHO Plus! I noticed an increase in my MPGs as well when I moved to Tucson and a decrease when I moved to Wisconsin. I believe it has more to do with air density than any other factor. Very humid air is denser. This creates more drag. Imagine swinging your hand in a pool and then out of one. Now imagine that at 70 MPH. HHI was a very humid place. Wisconsin is either humid or snowing. I am sure other factors could apply as well but over a year or so, those factors would go away (tire pressure, Computers, driving habits etc...) So long story short I would bet a 62" plasma TV with an authentic Donald Trump hair flip I am correct! Take that! Ha!!
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Reprogram Laura?

Older Hondas (and probably most cars) have a warning beeper that sounds when the key is in the ignition and the door open. This can become a headlight warning buzzer by adding a relay activated from any wire that is "hot" when the lights are on, and N/O contacts wired to ground a blue/white wire from the control unit. I just did it recently and it cost me 3 bucks for the relay. No more dead batteries!
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Mileage change

You blew it. I use trip meter to estimate my mileage all the time. Tire pressure, tread, and ethanol content of the gas all affect the mileage. Ethanol content changes in areas and seasons.
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Helen from SLC

I can't explain why the fuel economy increased after she moved from WI to SLC, but my guess would be driving habits (more highway miles, for example). She said she lost her additional mileage AFTER putting on new tires and has yet to get it back. Are the tires the sames size or are they different? Even if they are the same size, the new tires could be larger in diameter and now the speedometer is probably slow. In other words, the vehicle is now going faster than the speedometer reads. If this is true,then when the trip odometer reads 100 miles, the vehicle really traveled, say 110 miles, depending on how far off the speedometer is. Therefore, the fuel economy never changed, just the odometer. To verify this, she needs to drive down the highway and set the cruise on 60mph. While driving at this constant speed, time yourself between mile markers. It will take 60 seconds to travel 1 mile. If it takes you less than a minute, then you are traveling faster than 60mph and thus traveling farther than the odometer is reading. You can then figure the percentage that the speedometer is off, and calculate the loss in mileage per tank.
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Great Show but you missed it on the Salt Lake Toyota

Love the show as usual. There is 3600 feet of altitude difference between SLC and milwaukee. The ECU did not have time to adjust because it was towed to SLC. The short term MPG improvement on the 4X4 was likely because of short term self adjustment of the stock ECU to the altitude difference. She would not want to go back to the bad adjustments because it was likely running lean and causing short term damage to the engine.
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