# TIRE WEAR AFFECTS SPEED

Published Oct. 23, 2010|Updated Oct. 27, 2010

Q: It occurred to me the other day that as a car is driven and the tires wear down, each tire's circumference decreases. Since the speedometer measures speed through the revolutions of the drive shaft or something, and then converts that to miles per hour, isn't it based on an assumption that your tires have a certain circumference? And as the tire wears and the circumference decreases, won't the actual speed and the speed displayed on the speedometer eventually be different? My question is, how much does tread wear affect the speedometer reading? Is it significant? And can I use this argument to get out of a speeding ticket? Thanks. Todd

Tom: I was wondering where you were going with that question, Todd. Now I've got it.

Ray: We'll do the math for you. Let's say a typical new tire is 25 inches in diameter. And let's say there's half an inch of tread all the way around the outside. Therefore, when the tire goes from new to old, you'd lose an inch of diameter (half an inch on either side). So, in our scenario, a typical bald tire is 24 inches in diameter.

Tom: We know that the circumference of a circle is directly proportional to its diameter. So we can conclude that when you go from 25 to 24 inches in diameter, the change in the speedometer reading is 4 percent.

Ray: So the difference between a brand-new tire and a baldie changes your speedometer reading by less than 3 mph when you're going 65.

Tom: That's probably not enough to change a speeding ticket, Todd. Besides, it goes in the WRONG direction. As your tires shrink in size, the car is actually going slower than the speedometer reads, not faster. So while your speedometer read 80, your ticket only cited you for going 77. But you already knew that.

Ray: But you can try using the argument in traffic court anyway, Todd. Maybe you can flimflam them with the math. Just be careful they don't throw out your speeding ticket only to give you a citation for bald tires!

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Maintenance-free? Not so fast

Q: Please help me settle a bet with my father-in-law . . . the winner will buy the loser his next battery. My father-in-law claims that even though car batteries may be marked "maintenance-free," if the battery has vent caps you can pry off, you should do so, and check and - when necessary - refill the electrolyte levels as needed. I say no, that modern car batteries do not require checking electrolyte levels, and that by prying off the vent caps, you actually could cause some harm to a maintenance-free battery. Can you help settle our bet? Michael

Ray: We can, but you're not going to like the settlement, Michael.

Tom: Back in the 1970s, battery manufacturers switched over to calcium lead plates, because they seriously reduced outgassing.

Ray: Outgassing, aside from being a by-product of eating a beef-cheese-chorizo burrito, is when the fluid inside the battery heats up and "boils off." It's similar to what happens when water boils and gives off vapor - eventually, you run out of water.

Tom: Because calcium lead grids reduced outgassing significantly, and all but eliminated the need to add water to the battery cells, these batteries were called "maintenance-free."

Ray: But they don't eliminate outgassing completely. And if the engine compartment is subject to extremely high temperatures, even a maintenance-free battery can lose fluid.

Tom: With maintenance-free batteries, outgassing is most likely to happen in areas of the country that get extremely hot in the summer, or where cars are in constant stop-and-go conditions and their engine compartments get up to very high temperatures.

Ray: Most people will never need to add water to their maintenance-free batteries. In fact, the vast majority of batteries don't even allow you to check the fluid levels anymore. But if your maintenance-free battery does have removable caps, there certainly is no harm in removing them and adding water if the fluid happens to be low.

Tom: It's not a regular maintenance item, like it was 20 or 30 years ago, Michael. But you owe your pop-in-law his next battery. If you're really lucky, his next battery will be for his flashlight.