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Comment by stcredzero | Hacker News Reader

> I do recall setting the cruise control to about 54 m.p.h., as I wrote. The log shows the car traveling about 60 m.p.h. for a nearly 100-mile stretch on the New Jersey Turnpike. I cannot account for the discrepancy, nor for a later stretch in Connecticut where I recall driving about 45 m.p.h., but it may be the result of the car being delivered with 19-inch wheels and all-season tires, not the specified 21-inch wheels and summer tires.

Okay, someone please explain to me how tires with a smaller circumference are going to result in faster speeds?

John Broder already came across as someone who's hazy on basic physics. Maybe he's not so hot on geometry, either.

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well, assuming that the cruise control / rest of car was aware of the tire change, but the logging system wasn't

you have the motor spinning faster to make up for the smaller tires, while the logging system just sees the motor spinning faster.

however, as counterpoint to this, the read distance would probably change as well (unless the logging system was only partially aware). Then again there was some discrepancies with the travel distance as well, but that discrepancy isn't nearly enough to account for it.

I think here the case is simple. Broder did not record his speed accurately, he just drove "around 45 mph", which very well might be actually 55 mph, on a good road with a good car 10 mph difference in speed doesn't feel at all. However, he can't just say "I didn't really collect the data accurately" since it'd hurt his credibility, so he mentions completely unrelated discrepancy in hope that most of the readers would say "ok, it's plausible, maybe he's right, maybe he's not, we're not engineers, we don't know whose fault it is". Thus both Tesla's point would be neutralized and Broder's credibility won't be hurt.

Or maybe he did report what the speedometer showed, and the calibration is off.

That is a possibility too. On a mass-produced car, I'd evaluate this as pretty low probability, since setting speedometer for the actual car specs is a basic expectation and if any company would let wrongly calibrated cars out of their factory it would be a huge liability issue and a very loud scandal. However, if custom modifications were made to the car, it is not out of the question, theoretically. But then the effect of the smaller tires should be the opposite, not? I.e. with smaller tires the speedometer should overestimate the speed, not underestimate?

Maybe you aren't so hot either. Tell me, what do 19-inch and 21-inch specify? The outer diameter of the tire or the DIAMETER OF THE RIM?

> Maybe you aren't so hot either.

So, grade school geometry refresher. When you increase the diameter of a circle, what happens to its circumference?

Thanks for playing: "Do I think I am clever but just accidentally revealed I'm the level of dolt that needs basic geometry spelled out and assumes everyone else is as clueless as me?"

I do like the game :)

The overall diameter of the wheel is determined by the diameter of the rim + 2 times the thickness of the tire(!). Cars with larger rims usually have far thinner tires on them so the overall diameter stays almost the same. Whatever the case, the speedometer will need to be checked and adjusted. Human error in this case can swing either way.

> I do like the game :)

D'oh! You win. (Not in the game mentioned above, either. I'm willing to be shown as a bit of a tires noob, so long as you're not painting me as a rube who forgot grade school geometry. EDIT: Never bought aftermarket rims in my life!)

In any case, this is enough to explain a 7 or 8% difference in the apparent direction the logs show?

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