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Will changing the tires on my car affect the speedometer?

Posted by John Paul  January 31, 2012 12:44 PM

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Q. I have a 2003 Porsche Carrera with original wheels. The rear tire size is 285/30ZR -18. My question is, can I put 295/35ZR-18 tires and will it affect the accuracy of the speedometer?

A. The speedometer will read 3.5 percent slower than the actual speed. As an example when the speedometer is reading 58 miles per hour the car will actually be going 60 miles per hour. Personally, I would stay with the stock tire.

Q. I have a 2008 Toyota RAV4, about six months ago it starting having trouble starting when the engine was cold. Once it started in the morning it would be fine the rest of the day. I took it to the Toyota dealer who said they checked everything and could not find an issue. They told me I had to leave it overnight for them to duplicate the problem, is this necessary?

A. Leaving the car overnight, may be inconvenient but will certainly help the technician test and properly repair the vehicle. Finding an intermittent problem on a car that runs well can certainly be difficult and depending on the integrity of the shop could lead to expensive misdiagnosis.

Q. My 2004 Volvo XC-90 SUV died on me as I was leaving home. It had to be towed to the dealer, who diagnosed contaminated fuel but had difficulty determining the source of the problem until they spoke with the national Volvo Service Center. They were told that the plastic fuel tanks on older Volvos had experienced some deterioration from too much ethanol in fuel, thereby causing the fuel contamination. They also said that Volvo did not accept responsibility for a defective part, implying that the fuel problem not a Volvo problem. The repair will involve replacement of the tank and possibly other parts of the fuel system. Are there any Volvo service bulletins about this issue, or any online discussion groups that identify similar problems? I would appreciate any help you can provide to help solve this mystery.

A. I have heard of problems with plastic fuel tanks when the concentration of ethanol gets above 15 percent. Currently here in the northeast we have gasoline that has no more than 10 percent ethanol, so the tank deterioration shouldn’t have been caused by the fuel. I checked AllData, the technical database I use for research and didn’t find any service bulletins for this issue. I also checked a professional technician’s forum and again found nothing to indicate a problem with ethanol issues. I did find a bulletin about defective fuel pumps which could certainly causes the car to quit running.

Q. We own a 2001 Ford Taurus and the air conditioner stops working every few minutes. We had it recharged twice, but it still shuts off after a few minutes. We are in our 80’s and just want a little cool air when we get some hot weather.

A. If the system was recharged twice, that indicates there is a leak. When the system runs low on refrigerant the air conditioner system will only run for a few minutes. At this point I would want the system reevaluated and tested for a leak. Only once the leak is repaired will the system operate normally.

Q. After last year’s tropical storm I went 4 days without power. I can’t justify a generator but was wondering is there some way I can utilize my truck’s alternator to power some items in my house. I’m especially interested in keeping my refrigerator cold. Is this even conceivable?

A. You could consider a voltage inverter that hooks directly up to your truck’s battery. I used a 750 watt inverter to power a small refrigerator for several hours at a time at my house. These inverters put a fair bit of stress on the vehicle’s battery and the engine will need to be restarted to keep the battery charged. Before buying an inverter, check the wattage of the items that you want to power to find the appropriate sized unit.

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Clifford Atiyeh is an automotive writer and car enthusiast . He has spent his entire life driving cars he doesn't own.
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Bill Griffith is a veteran Boston Globe reporter, having reviewed cars for more than 10 years and serving as assistant sports editor for 25 years. He was also the paper's sports media columnist.
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AAA's Car Doctor, John Paul John Paul is public affairs manager for AAA Southern New England, a certified mechanic, and a Globe columnist. He hosts a weekly radio show on WROL.
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