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AUTO Q AND A: Discrepancy in speed? Trust GPS reading

Question: I drive a '97 Acura RL. When my speedometer reads 70 miles per hour, my Garmin GPS readout shows 66. I think the factory readout is set a few mph less than actual speed, but I don't want to get a ticket. Which one is correct, the speedo...

Question: I drive a '97 Acura RL. When my speedometer reads 70 miles per hour, my Garmin GPS readout shows 66. I think the factory readout is set a few mph less than actual speed, but I don't want to get a ticket. Which one is correct, the speedometer or the GPS? I'm thinking the GPS. The car has custom 16-inch wheels, but they are the original equipment size, so they would not affect the speedometer reading.

Answer: I'm betting the GPS is more accurate. I have the same situation with two different GPS units in two different vehicles; both GPS units indicate a couple of miles per hour slower than the speedometer reads. The GPS units, which calculate speed and position by monitoring at least three geosynchronous satellites, are incredibly accurate.

The speedometer in any automobile is completely dependent upon the rolling diameter of the tires. As tires wear, their diameter decreases. Even temperature and pressure changes in a tire affect its rolling diameter to some degree.

For example, a P215/60-16 tire with a diameter of 26.2 inches will lose a quarter-inch of tread height and a half-inch in rolling diameter as its tread wears down from 10/32 inch to 2/32 inch. That's about a 2 percent diameter reduction in diameter over the life of the tire, which will affect speed accuracy. Two percent at 70 mph is 1.4 mph, meaning the vehicle would be traveling 1.4 mph slower than the speedometer read when the tires were new.

Standards for speedometer accuracy allow a margin of error of 5 percent at highway speeds, meaning at 70 mph an error of up to 3.5 mph is OK. Believe the GPS.

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Q: I have a 2003 Honda CR-V with all-wheel drive. At 60,000 miles, my Honda dealer told me I needed to replace the front struts because they were leaking. The cost was almost $800. I did not note any leaking by lying under the car using a flashlight, and there was no oil on my garage floor either. I agreed only because the car was still under warranty and I would have to pay only a $50 deductible.

A: There are two ways to look at it. Unless you noticed a marked change in ride quality, handling and steering response, I would be surprised to find worn or leaking struts on your vehicle. On the other hand, if you're going to keep your vehicle for its full life, you'll likely need to replace the shocks or struts once -- and if the warranty was expired, you'd end up paying much more than $50.

Q: I have a 2000 Ford F-150 with 69,000 miles. I don't get much heat from it. The temperature does not reach the "low" end of the gauge. Could this be a thermostat problem, or something more serious?

A: I hope it's the thermostat failing to close properly and regulate coolant operating temperature. But make sure there's no loss of coolant that could indicate a leaking head gasket or intake manifold gasket.

Q: We bought a used '07 Mazda 6 that is still under warranty. I will do the oil changes myself. What do I need to prove the maintenance has been done? Also, we do not have records from before, is that an issue?

A: Just keep receipts for the maintenance items like oil and filters you purchase. Keep a vehicle logbook with mileage, fuel purchases, maintenance and repairs. Mazda dealers will have service records for the car if it was serviced by a dealer before you bought it.

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