Just stumbled across this amusing little exchange in a Canadian newspaper, The Province. The question about bald tyres comes from a guy named Todd and the reply comes from legendary US motoring experts, the brothers Tom and Ray Magliozzi (also known in ‘showbiz’ circles as Click and Clack).
“Dear Tom and Ray – It occurred to me the other day that as a car is driven and the tyres wear down, each tyre’s circumference decreases.
“Since the speedometer measures speed through the revolutions of the driveshaft or something, and then converts that to miles per hour, isn’t it based on an assumption that your tyres have a certain circumference? And as the tyre 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?
To which the motoring duo reply:
“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 tyre 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 tyre 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 tyre 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 four per cent.
“RAY: So the difference between a brand-new tyre and a baldie changes your speedometer reading by less than three miles per hour 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 tyres shrink in size, the car is actually going slower than the speedometer reads, not faster.
“RAY: But you can try using the argument in traffic court anyway, Todd. Maybe you can flim-flam them with the math. Just be careful they don’t throw out your speeding ticket only to give you a citation for bald tyres!”
Denna Bowman, Head Office