odometer/speedometer accuracy
10-08-12, 02:44 AM
#16
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stevesxm, I think you missed the points I was trying to make for the question I was trying to answer. You make some of the same points as myself yet discard some of your own calculations to say it doesn't make a difference. I say they do and it adds up. I can further the points I was trying to make without even having to go outside the two OEM tire size choices for the RX.
Take a look at the Yokohama Prada-X in the OEM sizes for 17" (225/65R17) and 18" (235/55R18) tires. Listed as having a half inch diameter difference when new and a difference of twelve revolutions per mile.
http://www.tirerack.com/tires/tires....wtpackage=true
This does not take into consideration tire flex due to the sidewall construction. Will throw that out for now. FWIW, the revolutions per mile numbers above for the Prada take into consideration sidewall flex and TireRack notes this is not a calculated number:
http://www.tirerack.com/tires/tirete...jsp?techid=203
Thread depth for each size is 11/32". If you run them to 3/32" you would lose 8/32" off each end of the diameter for a total of 16/32" or 0.5". For the 17" tire, the total diameter would drop from 28.6" to say 28.1". For the 18" tire the diameter would drop from 28.1" to say 27.6" From max value to min value for the OEM tire sizes the tire diameter needing to be considered might vary from 28.6" to 27.6". One full inch of the diameter. A new 17" tire would need approx. 705.2 revs/mile while a worn 18" tire would need 730.7 revs/mile. For argument sake the worn 17" tire would need 717.7 revs/mile. Gives us values of 1.8% difference over the life of a 17" tire, the same for the difference between new 17" and 18" tires, and a potential 3.5% difference between the min and max values on OEM tire sizes over their lifetime.
So, what am I getting at...
3.5% is quite a bit to compensate for here. Your vehicle does not know if you have new 17" tires installed or used 18" tires. This is one point I was making. The base and +1 tire sizes are just not close enough in diameter. This possible 2+ MPH difference (at 65MPH and nearly 3MPH difference at 80MPH in TX) cannot be discarded. The manufacturer has to take this into consideration during the design process making the speedometer and odometer to be compromises at best and brings a particular bias to the equation depending on the company. While they may approximate very closely at one extreme, they will be off on the other. As they do not want to understate your speed for legal reasons (it would be OK to overstate your speed) that also factors in as well.
Depending on the choices Lexus/Toyota decided to make with the RX regarding speedometer and odometer accuracy -- when, where and how much -- plus which tire you select and its size will affect the overall accuracy of these systems. And, during the life of your tires you can estimate on 1MPH off from beginning to the end just due to tire wear. All these affect distance calculations. They affect MPG calculations. Will your GPS exactly match the distance traveled according to your odometer? Over a long distance, probably not.
In this example I decided to throw out any affect of sidewall flex. I also did not take into consideration any information from other tires other than the Prada. They just happen to have listed the specs for the two sizes needed. Selecting different tires from different manufacturers may exacerbate this problem or it may bring the numbers closer together.
Take a look at the Yokohama Prada-X in the OEM sizes for 17" (225/65R17) and 18" (235/55R18) tires. Listed as having a half inch diameter difference when new and a difference of twelve revolutions per mile.
http://www.tirerack.com/tires/tires....wtpackage=true
This does not take into consideration tire flex due to the sidewall construction. Will throw that out for now. FWIW, the revolutions per mile numbers above for the Prada take into consideration sidewall flex and TireRack notes this is not a calculated number:
http://www.tirerack.com/tires/tirete...jsp?techid=203
Thread depth for each size is 11/32". If you run them to 3/32" you would lose 8/32" off each end of the diameter for a total of 16/32" or 0.5". For the 17" tire, the total diameter would drop from 28.6" to say 28.1". For the 18" tire the diameter would drop from 28.1" to say 27.6" From max value to min value for the OEM tire sizes the tire diameter needing to be considered might vary from 28.6" to 27.6". One full inch of the diameter. A new 17" tire would need approx. 705.2 revs/mile while a worn 18" tire would need 730.7 revs/mile. For argument sake the worn 17" tire would need 717.7 revs/mile. Gives us values of 1.8% difference over the life of a 17" tire, the same for the difference between new 17" and 18" tires, and a potential 3.5% difference between the min and max values on OEM tire sizes over their lifetime.
So, what am I getting at...
3.5% is quite a bit to compensate for here. Your vehicle does not know if you have new 17" tires installed or used 18" tires. This is one point I was making. The base and +1 tire sizes are just not close enough in diameter. This possible 2+ MPH difference (at 65MPH and nearly 3MPH difference at 80MPH in TX) cannot be discarded. The manufacturer has to take this into consideration during the design process making the speedometer and odometer to be compromises at best and brings a particular bias to the equation depending on the company. While they may approximate very closely at one extreme, they will be off on the other. As they do not want to understate your speed for legal reasons (it would be OK to overstate your speed) that also factors in as well.
Depending on the choices Lexus/Toyota decided to make with the RX regarding speedometer and odometer accuracy -- when, where and how much -- plus which tire you select and its size will affect the overall accuracy of these systems. And, during the life of your tires you can estimate on 1MPH off from beginning to the end just due to tire wear. All these affect distance calculations. They affect MPG calculations. Will your GPS exactly match the distance traveled according to your odometer? Over a long distance, probably not.
In this example I decided to throw out any affect of sidewall flex. I also did not take into consideration any information from other tires other than the Prada. They just happen to have listed the specs for the two sizes needed. Selecting different tires from different manufacturers may exacerbate this problem or it may bring the numbers closer together.
in the real world the tires and the tires options that can actually fit on the car aren't going to make any significant real world difference . if you are going 80 you are going to get a ticket whether or not its 79.6 or 81.3
and a couple other things to consider. first of all who says these things are wrong at all ? run a real test with some scientific standards and get back to me....
i would be willing to bet there are some stringent standards an oem manufacturer has to me for accuracy and an oem like lexus risks getting their butts handed to them by the feds if they sytematically and willfully violate them .
can you screw up your readings with tall tires ? yes. but they have to be meaningfully taller AND fit on the car. thats not the case here.
addendum... a quick search suggests there are two relevant federal standards... one from 97 that mandates an error no GREATER than 5 % and another in 02 that further clarifies odometer standards. honda was subject to a class action suit that forced them to recalibrate all their cars to a higher standard around that time. further the statute mandates that the error can NEVER be negative in so much as the the indicated speed can NEVER be less than the true speed. this info from a google search and various sources and not directly from any govt web site.
in this specific case i would suggest that whether its 1.5 % or 3 % it is well within tolerance for everyday use.
Last edited by stevesxm; 10-08-12 at 04:36 AM.
10-08-12, 08:15 AM
#17
Lexus Champion
Yup, same with mine^^^^^. Reads about 1.5-2.0 more mph than what the gps, and those digital road side speed`s the police set up around here.
10-08-12, 08:16 AM
#18
Rookie
Thread Starter
stevesxm, I think you missed the points I was trying to make for the question I was trying to answer. You make some of the same points as myself yet discard some of your own calculations to say it doesn't make a difference. I say they do and it adds up. I can further the points I was trying to make without even having to go outside the two OEM tire size choices for the RX.
Take a look at the Yokohama Prada-X in the OEM sizes for 17" (225/65R17) and 18" (235/55R18) tires. Listed as having a half inch diameter difference when new and a difference of twelve revolutions per mile.
http://www.tirerack.com/tires/tires....wtpackage=true
This does not take into consideration tire flex due to the sidewall construction. Will throw that out for now. FWIW, the revolutions per mile numbers above for the Prada take into consideration sidewall flex and TireRack notes this is not a calculated number:
http://www.tirerack.com/tires/tirete...jsp?techid=203
Thread depth for each size is 11/32". If you run them to 3/32" you would lose 8/32" off each end of the diameter for a total of 16/32" or 0.5". For the 17" tire, the total diameter would drop from 28.6" to say 28.1". For the 18" tire the diameter would drop from 28.1" to say 27.6" From max value to min value for the OEM tire sizes the tire diameter needing to be considered might vary from 28.6" to 27.6". One full inch of the diameter. A new 17" tire would need approx. 705.2 revs/mile while a worn 18" tire would need 730.7 revs/mile. For argument sake the worn 17" tire would need 717.7 revs/mile. Gives us values of 1.8% difference over the life of a 17" tire, the same for the difference between new 17" and 18" tires, and a potential 3.5% difference between the min and max values on OEM tire sizes over their lifetime.
So, what am I getting at...
3.5% is quite a bit to compensate for here. Your vehicle does not know if you have new 17" tires installed or used 18" tires. This is one point I was making. The base and +1 tire sizes are just not close enough in diameter. This possible 2+ MPH difference (at 65MPH and nearly 3MPH difference at 80MPH in TX) cannot be discarded. The manufacturer has to take this into consideration during the design process making the speedometer and odometer to be compromises at best and brings a particular bias to the equation depending on the company. While they may approximate very closely at one extreme, they will be off on the other. As they do not want to understate your speed for legal reasons (it would be OK to overstate your speed) that also factors in as well.
Depending on the choices Lexus/Toyota decided to make with the RX regarding speedometer and odometer accuracy -- when, where and how much -- plus which tire you select and its size will affect the overall accuracy of these systems. And, during the life of your tires you can estimate on 1MPH off from beginning to the end just due to tire wear. All these affect distance calculations. They affect MPG calculations. Will your GPS exactly match the distance traveled according to your odometer? Over a long distance, probably not.
In this example I decided to throw out any affect of sidewall flex. I also did not take into consideration any information from other tires other than the Prada. They just happen to have listed the specs for the two sizes needed. Selecting different tires from different manufacturers may exacerbate this problem or it may bring the numbers closer together.
Take a look at the Yokohama Prada-X in the OEM sizes for 17" (225/65R17) and 18" (235/55R18) tires. Listed as having a half inch diameter difference when new and a difference of twelve revolutions per mile.
http://www.tirerack.com/tires/tires....wtpackage=true
This does not take into consideration tire flex due to the sidewall construction. Will throw that out for now. FWIW, the revolutions per mile numbers above for the Prada take into consideration sidewall flex and TireRack notes this is not a calculated number:
http://www.tirerack.com/tires/tirete...jsp?techid=203
Thread depth for each size is 11/32". If you run them to 3/32" you would lose 8/32" off each end of the diameter for a total of 16/32" or 0.5". For the 17" tire, the total diameter would drop from 28.6" to say 28.1". For the 18" tire the diameter would drop from 28.1" to say 27.6" From max value to min value for the OEM tire sizes the tire diameter needing to be considered might vary from 28.6" to 27.6". One full inch of the diameter. A new 17" tire would need approx. 705.2 revs/mile while a worn 18" tire would need 730.7 revs/mile. For argument sake the worn 17" tire would need 717.7 revs/mile. Gives us values of 1.8% difference over the life of a 17" tire, the same for the difference between new 17" and 18" tires, and a potential 3.5% difference between the min and max values on OEM tire sizes over their lifetime.
So, what am I getting at...
3.5% is quite a bit to compensate for here. Your vehicle does not know if you have new 17" tires installed or used 18" tires. This is one point I was making. The base and +1 tire sizes are just not close enough in diameter. This possible 2+ MPH difference (at 65MPH and nearly 3MPH difference at 80MPH in TX) cannot be discarded. The manufacturer has to take this into consideration during the design process making the speedometer and odometer to be compromises at best and brings a particular bias to the equation depending on the company. While they may approximate very closely at one extreme, they will be off on the other. As they do not want to understate your speed for legal reasons (it would be OK to overstate your speed) that also factors in as well.
Depending on the choices Lexus/Toyota decided to make with the RX regarding speedometer and odometer accuracy -- when, where and how much -- plus which tire you select and its size will affect the overall accuracy of these systems. And, during the life of your tires you can estimate on 1MPH off from beginning to the end just due to tire wear. All these affect distance calculations. They affect MPG calculations. Will your GPS exactly match the distance traveled according to your odometer? Over a long distance, probably not.
In this example I decided to throw out any affect of sidewall flex. I also did not take into consideration any information from other tires other than the Prada. They just happen to have listed the specs for the two sizes needed. Selecting different tires from different manufacturers may exacerbate this problem or it may bring the numbers closer together.
10-08-12, 12:25 PM
#19
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if the speedometer issue is meaningful for some reason you can adjust that slightly with tire sizing but not very much because the clearence issue limits your options. if you actually know your error to some degree of scientific accuracy with your existing tires, you can actually put on a taller tire from whatever manufacturer offers it and that will put your true speed more in line with your indicated speed. it seems like a lot of work and money to go thru just for that when you could just look down at 57 mph and already know you are really going 55 ... but some things are more important to people than others...
and there is no assurance that the odometer is wrong as well if the speedo is. several oem's ( i discovered) have a different algorythm in their software so that the speedo reads a bit high as mandated by the feds but the odometer is not scaled the same way. i don't know nor could discover what lexus does in this regard but the possibility is that you could put a taller tire on to fix the speedo and actually MAKE the odo incorrect, i suppose.
10-08-12, 05:53 PM
#20
No, I am saying that the whole system is a compromise and may not very accurate at all. It may or may not be more accurate depending on your choice of tire and size, however, that too is very subjective. The system's accuracy has more to do with choices the manufacturer makes than anything you have control over. Plus, on top of that the speedometer and odometer will drift in accuracy over the lifetime of set of tires -- fractionally, however, still significant from beginning to end.
10-08-12, 06:37 PM
#21
well... as you note, there are certainly all sort of tires sizes available. but having said that , the realities of what you can actually bolt on the car without torching the fenders off or seizing them against the strut actually has a bit more to do with the question.
in the real world the tires and the tires options that can actually fit on the car aren't going to make any significant real world difference . if you are going 80 you are going to get a ticket whether or not its 79.6 or 81.3
and a couple other things to consider. first of all who says these things are wrong at all ? run a real test with some scientific standards and get back to me....
i would be willing to bet there are some stringent standards an oem manufacturer has to me for accuracy and an oem like lexus risks getting their butts handed to them by the feds if they sytematically and willfully violate them .
can you screw up your readings with tall tires ? yes. but they have to be meaningfully taller AND fit on the car. thats not the case here.
addendum... a quick search suggests there are two relevant federal standards... one from 97 that mandates an error no GREATER than 5 % and another in 02 that further clarifies odometer standards. honda was subject to a class action suit that forced them to recalibrate all their cars to a higher standard around that time. further the statute mandates that the error can NEVER be negative in so much as the the indicated speed can NEVER be less than the true speed. this info from a google search and various sources and not directly from any govt web site.
in this specific case i would suggest that whether its 1.5 % or 3 % it is well within tolerance for everyday use.
in the real world the tires and the tires options that can actually fit on the car aren't going to make any significant real world difference . if you are going 80 you are going to get a ticket whether or not its 79.6 or 81.3
and a couple other things to consider. first of all who says these things are wrong at all ? run a real test with some scientific standards and get back to me....
i would be willing to bet there are some stringent standards an oem manufacturer has to me for accuracy and an oem like lexus risks getting their butts handed to them by the feds if they sytematically and willfully violate them .
can you screw up your readings with tall tires ? yes. but they have to be meaningfully taller AND fit on the car. thats not the case here.
addendum... a quick search suggests there are two relevant federal standards... one from 97 that mandates an error no GREATER than 5 % and another in 02 that further clarifies odometer standards. honda was subject to a class action suit that forced them to recalibrate all their cars to a higher standard around that time. further the statute mandates that the error can NEVER be negative in so much as the the indicated speed can NEVER be less than the true speed. this info from a google search and various sources and not directly from any govt web site.
in this specific case i would suggest that whether its 1.5 % or 3 % it is well within tolerance for everyday use.
What does all this mean in the bigger picture? How, when and where can you make the system completely accurate? Fix one problem here and it is off some place else. If it is accurate with the 17" tires when new it will be off if the customer takes the +1 upgrade. Heck, even if you mandate that only one model/size/brand of tire can only be used you cannot make the system completely accurate. If it is accurate with new tires it will not be accurate come the time they need to be replaced.
It is safe to assume you are not going faster than the speedometer says and in many cases 1-3 MPH slower depending on your speed and other factors such as those I have mentioned previous. It is just something you just live with.
10-08-12, 07:39 PM
#22
Rookie
Thread Starter
OK, I can live with my speedometer not being accurate, as long as it doesn't affect the odometer reading. Does it?
Is it possible to ask the dealer to do a speedometer/odometer check? Is there such a thing as an odometer test that they can do in the shop? My concern is the car saying it has more miles than actually happened, as that affects terms of warranty.
Is it possible to ask the dealer to do a speedometer/odometer check? Is there such a thing as an odometer test that they can do in the shop? My concern is the car saying it has more miles than actually happened, as that affects terms of warranty.
10-08-12, 10:02 PM
#23
OK, I can live with my speedometer not being accurate, as long as it doesn't affect the odometer reading. Does it?
Is it possible to ask the dealer to do a speedometer/odometer check? Is there such a thing as an odometer test that they can do in the shop? My concern is the car saying it has more miles than actually happened, as that affects terms of warranty.
Is it possible to ask the dealer to do a speedometer/odometer check? Is there such a thing as an odometer test that they can do in the shop? My concern is the car saying it has more miles than actually happened, as that affects terms of warranty.
Lots of reading on the Internet on these topics. Feel free to search on your own. Here are a few in the meantime:
http://www.wcnc.com/on-tv/Going-the--115241069.html
http://en.wikipedia.org/wiki/Speedometer
If you really want to warp your mind, try reading the following:
http://uscode.house.gov/download/pls/49C327.txt
Here is some information on the Honda problem that stevesxm mentioned previous:
http://autos.aol.com/article/honda-odometer-problem/
FWIW, most dealerships will not touch this topic with a 10 ft. pole and I do not blame them. This, IMO, is a mainly non-issue. Move along... there is nothing to see here. There are plenty of other more important things to worry about in life.
10-10-12, 07:16 AM
#24
Lexus Test Driver
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I got a GPS speedo for my phone and popped it on my dash. The GPS is quite accurate (I believe) and the speedo and the GPS was only about 2km's out from about 100km/h onwards. So when my car is showing 100 on the needle, the GPS is showing about 100 as well. At 112km/h on the speedo the GPS shows 110.
03-22-14, 09:13 AM
#25
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lexus RX350 speedometer registering fast
It seems to me that my 2009 RX350 and the 2009 Toyota Corolla both suffer from the same deficiency. I have checked them repeatedly using my GPS and the Lexus is off my 3 MPH. My 2009 Corolla had the same problem. Now when I drive I use my GPS to make sure of my speed. I find it interesting that two models produced by the same company have this problem and I doubt that it is just and anomaly. While Toyota may be keeping me safe by driving slower I suspect it also allows them to clam better mileage. Any thought?:
03-22-14, 09:41 AM
#26
It seems to me that my 2009 RX350 and the 2009 Toyota Corolla both suffer from the same deficiency. I have checked them repeatedly using my GPS and the Lexus is off my 3 MPH. My 2009 Corolla had the same problem. Now when I drive I use my GPS to make sure of my speed. I find it interesting that two models produced by the same company have this problem and I doubt that it is just and anomaly. While Toyota may be keeping me safe by driving slower I suspect it also allows them to clam better mileage. Any thought?:
No, MPG numbers are determined under set rules by the EPA and Toyota/Lexus could care less what you get for your mileage. They just need to get close. And all vehicles are like this to one degree or another. Technology can get us closer, however, there would need to be constant calibration.
If you have the time, you can read my tomes above. In short, the speedometer will never say you are going slower than you are. Would put Toyota/Lexus in hot water and there would be many lawsuits. Also, as your tires wear you are going even slower compared to your speedometer.
This is not just a Toyota/Lexus problem. I have seen German vehicles whose speedometer was off even more.
Once the situation is understood, why all the fuss and why does all this matter? Granted, I am an old man now. I go fast when i want to go fast and I go slow when I want to go slow. Am happy that speedometers are like this. When I am in neighborhoods where kids might be playing I am glad to know I am going slower than the posted speed limit. On the highways, everyone is going faster than the posted limits regardless. So again, why does this matter?
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