Tire circumference redux redux...
in [Toyota]
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From: Jeff Strickland on 15 Nov 2009 12:48 "Tegger" <invalid(a)invalid.inv> wrote in message news:Xns9CC3B81E290A8tegger(a)208.90.168.18... > "Jeff Strickland" <crwlrjeff(a)yahoo.com> wrote in > news:hdmqpo$s17$1(a)news.eternal-september.org: > >> >> "Tegger" <invalid(a)invalid.inv> wrote in message >> news:Xns9CC3551732D7Ctegger(a)208.90.168.18... >>> Again due to the age of the original threads, I'm going to start a >>> new one to try and summarize what the older threads appear to have >>> flushed out. And >>> I was more wrong than right in my original assertions, apparently. >>> >>> > > > <selective snipping in effect> > > > >>> - Different tire sizes can indeed make a difference to your >>> speedometer. >>> >> >> Of course they do. Is the difference significant? That's the question. > > > > > You never read my revision, evidently. > > Yes, I did. But I had clicked Send first. > >> >> >> Granted, a low air pressure condition will affect the diameter, so any >> calculation should assume properly inflated tires. Having said that, a >> single low-pressure tire will not affect the speedometer -- it will >> affect something, but not the speedo. > > > > It /will/ affect the speedometer if it's on the "solid" side of the > diff. > > No more that turning a corner will affect the speedo. > >> >> >> >>> - Tire makers' reported revolutions-per-mile appear to be fairly >>> close to what would be experienced in the real world. >>> >> >> Of course they do, they calculate it from the numbers stamped on the >> sidewall. > > > > I wonder about that. > > C. E. White posted a bunch of tire sizes and revs-per-mile. They were > grouped around 3%, but they were quite obviously NOT straight > calculations from the nominal circumference. I suspect (prompted by > White) that belt placement relative to tread OD has something to do with > it. > > > >> >> >> >>> - When buying tires that are not of the size indicated on the sticker >>> that comes with your car, one needs to buy a tire that has the same >>> number of revolutions-per-mile as the original size in order to >>> retain speedometer accuracy. >>> >> >> That assumes the speedometer is accurate with the tires it has on it >> already. > > > > The factory is likely to make the speedometer slightly "fast" for > liability and marketing reasons. Do you know just how "fast"? Nor do I. > We're all guessing as to the inherent accuracy of the speedometer and > how aftermarket-sized tires may affect that. > > If the aftermarket selection is a size that's on the placard, the speedo will remain within its specification of reporting more speed than there really is. If the car has the base tires and is fitted with a size from one of the options packages, the speedometer accuracy will be improved, but for any practical matter it is really unaffected. The difference is insignificant. In order for an oversized tire to affect the speedo, the tire would likely not physically fit the car -- it would strike the fenders or the undercarriage. Unless the wheel openings were very large in relation to the "proper" sized tires, putting tires on a car that caused a significant change tothe speedometer would not fit. And, it the speedometer was impacted in any significant way, the gearing would be affected so strongly as to make the driving experience completely unenjoybalbe. And, yes, the factory is going to make the speedo fast -- it will read 60 at 56, not 56 at 60. The error rate is roughly in the 6% range. If the new tire is _slightly_ larger (but still one from the placard) then the speedometer error will be reduced to a range of about 3%. |