Here’s the latest reader question, along with my reply!
Paul asks: I once saw, but lost track of a formula (maybe an app now?) to apply to figure out the impact of the increase of tire sizes on speedometer readings. Do you have anything like that? I once figured it out longhand but misplaced my notes. Thanks. Also, great article on rim size and mpg.
My reply: Here’s a link to a calculator which will give you the correction. Keep in mind, though, that factory speedometers can be off even with the factory wheels/tires and everything else. This goes for both newer and older cars.
I have a GPS device I use when I test drive new cars and among other things, it gives me a speed readout. This often varies by as much as 2-3 MPG indicated. And this is why armed government workers have “calibrated” speedometers in their vehicles – the better to ticket you with!
On the upside, it’s often still a viable defense (depending on the judge) to present evidence that your speedometer was reading slower than you were actually driving and thus you had no idea, your honor, that you were speeeeeeeeding!
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Got a question about cars – or anything else? Click on the “ask Eric” link and send ’em in!
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