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From: Tegger on 14 Nov 2009 08:23 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. - Different tire sizes can indeed make a difference to your speedometer. - What matters for speedometer readings is the reported number of revolutions-per-mile, not the nominal diameter as calculated from the tire size as written on the sidewall. - Tire makers' reported revolutions-per-mile appear to be fairly close to what would be experienced in the real world. - 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. - The reduction in actual rolling circumference (from that which is calculated from the size that's written on the firewall) appears to be on the order of 3%. - Load on the tire also affects rolling circumference, but by less than one percent up or down. -- Tegger
From: Tegger on 14 Nov 2009 11:40 Tegger <invalid (a)invalid.inv> wrote innews: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. > > <snip> I'm slightly revising the previous message's points for clarity's sake: - Different tire sizes can indeed make a difference to your speedometer, even if they are the same nominal circumference. Those differences will be on the order of 1% or so. - What matters for speedometer readings is the reported number of revolutions-per-mile, not the nominal circumference as calculated from the tire size as written on the sidewall. - Tire makers' reported revolutions-per-mile appear to be fairly close to what would be experienced in the real world. - When buying tires that are not of the model or 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 model and size of tire in order to retain speedometer accuracy. - The reduction in actual rolling circumference (i.e.: down from that which is calculated from the size that's written on the sidewall) appears to be on the order of 3%. - Load on the tire also affects rolling circumference, but by less than one percent up or down. -- Tegger
From: Fatter Than Ever Moe on 14 Nov 2009 12:29 Tegger wrote: > Tegger <invalid (a)invalid.inv> wrote in> 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. >> >> > > <snip> > > > > > > I'm slightly revising the previous message's points for clarity's sake: > > > > - Different tire sizes can indeed make a difference to your speedometer, > even if they are the same nominal circumference. Those differences will be > on the order of 1% or so. > > - What matters for speedometer readings is the reported number of > revolutions-per-mile, not the nominal circumference as calculated from > the tire size as written on the sidewall. > > - Tire makers' reported revolutions-per-mile appear to be fairly close > to what would be experienced in the real world. > > - When buying tires that are not of the model or 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 model and > size of tire in order to retain speedometer accuracy. > > - The reduction in actual rolling circumference (i.e.: down from that > which is calculated from the size that's written on the sidewall) > appears to be on the order of 3%. > > - Load on the tire also affects rolling circumference, but by less than > one percent up or down. > > > And a GPS will tell you your true actual speed and keep Johnny Giveaticket off your butt.
From: Jeff Strickland on 14 Nov 2009 12:52 "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. > > > - Different tire sizes can indeed make a difference to your speedometer. > Of course they do. Is the difference significant? That's the question. My car takes a 225/55x15, but I changed it to a 225/45x17 and the speedometer error was corrected almost entirely. My truck takes a 255/65x16, but the previous owner had 265/75x16's installed, and the speedometer error is amplified. In the former (the case of my car), the affect speedometer is insignificant, and is infact an improvement over the factory tire fitment. In the latter (my truck), the affect on the speedo is not only significant, it is adverse if one hopes that observiance of the indicated speed will preculde a citation for exceeding the maximum speed allowed. > - What matters for speedometer readings is the reported number of > revolutions-per-mile, not the nominal diameter as calculated from the tire > size as written on the sidewall. > For all practical purposes, the revolutions per mile is accurately calculated by using the tire size numbers stamped on the sidewall of the tire. The size will tell the diameter to within a fraction of an inch, and the circimference can then be found, which gives the information needed to calculate the revolutions per mile. 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. > - 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. > - 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. If the car takes any of several different sizes -- option packages from the factory will offer different tires -- then any of the tires on the placard can be used on the car. One should carefully assess putting on tires that are not listed on the placard because the very real possibility is that the tire selection can physically strike some portion of the body or undercarriage. If a car comes with a placard that lists several tire options, the largest option will give the most accurate speedometer readings. The speedometer is required to read a speed that is ABOVE the actual speed. You cannot get a ticket for doing 55 in a 45 and use the defense that the speedo was only reading 40. If the speedo says 40, you will be doing 40 or less if the tires on the car are among the sizes listed on the placard.
From: Tegger on 14 Nov 2009 18:07
"Jeff Strickland" <crwlrjeff (a)yahoo.com> wrote innews: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. > > > 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. > > > >> - 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. -- Tegger |