Cooper S Built-in Speedo ERROR of 4% n OE Tire/Wheels
I'm more worried about the cars around me wrecking than about the speedo being a tad off
5th Gear
Joined: Feb 2006
Posts: 797
Likes: 1
From: West Coast, USA
Y'all are geeks! And I love it!
Accurate.
http://en.wikipedia.org/wiki/Accurate
Accuracy is the degree of veracity while precision is the degree of reproducibility.
http://en.wikipedia.org/wiki/Accurate
Accuracy is the degree of veracity while precision is the degree of reproducibility.
I think we can assume a linear relationship between number of revolutions of the wheel and gas consumption: 1 revolution = X consumption; 2 revolutions = 2X consumption. therefore 4% change in diameter = 4% change in revolutions = 4% change in gas consumption (ignoring K constant factors such as idling, exponential efect of drag as speed increases above 50 mph, etc.)
obviously we can make the 4% back by avoiding left turns where possible (as UPS has done for a multi-million dollar saving on gas).
does anyone think my comment is too geeky? please let me know.
obviously we can make the 4% back by avoiding left turns where possible (as UPS has done for a multi-million dollar saving on gas).
does anyone think my comment is too geeky? please let me know.
5th Gear
Joined: Nov 2003
Posts: 628
Likes: 0
From: Indianapolis
Just out of curiousity, is the concern over speedometer accuracy based on fuel economy concerns, the desire to avoid breaking speed limits, or just the desire to know exactly how fast you are going?
The thread seems to have wandered over towards the fuel economy side, in which case I would argue that the speedometer accuracy is far less important that other factors.
If the concern is to avoid breaking speed limits, use any of the stock size tires and you won't get a ticket if the speedomenter reading matches the posted speed limit.
If the desire is to know exactly how fast you are going, then you have problems. Even if you get it exactly right for any of the stock size tires (or any other for that matter). Between new and legally worn out, the tire diameter will decrease by .5" (approximately 2%). How often are should someone replace their tires, or get the speedo recalibrated?
Scott
90SM
The thread seems to have wandered over towards the fuel economy side, in which case I would argue that the speedometer accuracy is far less important that other factors.
If the concern is to avoid breaking speed limits, use any of the stock size tires and you won't get a ticket if the speedomenter reading matches the posted speed limit.
If the desire is to know exactly how fast you are going, then you have problems. Even if you get it exactly right for any of the stock size tires (or any other for that matter). Between new and legally worn out, the tire diameter will decrease by .5" (approximately 2%). How often are should someone replace their tires, or get the speedo recalibrated?
Scott
90SM
OVERDRIVE
Joined: May 2003
Posts: 7,926
Likes: 41
From: Greensboro, NC
It's mostly a good thing for the wallet that the Mini speedo is ambitious...
However, it is comforting to know exactly what my exact speed is via GPS so when an ocifer is tailgating me, I know I'm not driving too fast or too slow...either of which can get an unwanted light show from behind...in my case, I'd be nearly 5mph slower than the speedo reading...
However, it is comforting to know exactly what my exact speed is via GPS so when an ocifer is tailgating me, I know I'm not driving too fast or too slow...either of which can get an unwanted light show from behind...in my case, I'd be nearly 5mph slower than the speedo reading...
Just out of curiousity, is the concern over speedometer accuracy based on fuel economy concerns, the desire to avoid breaking speed limits, or just the desire to know exactly how fast you are going?
The thread seems to have wandered over towards the fuel economy side, in which case I would argue that the speedometer accuracy is far less important that other factors.
If the concern is to avoid breaking speed limits, use any of the stock size tires and you won't get a ticket if the speedomenter reading matches the posted speed limit.
If the desire is to know exactly how fast you are going, then you have problems. Even if you get it exactly right for any of the stock size tires (or any other for that matter). Between new and legally worn out, the tire diameter will decrease by .5" (approximately 2%). How often are should someone replace their tires, or get the speedo recalibrated?
Scott
90SM
The thread seems to have wandered over towards the fuel economy side, in which case I would argue that the speedometer accuracy is far less important that other factors.
If the concern is to avoid breaking speed limits, use any of the stock size tires and you won't get a ticket if the speedomenter reading matches the posted speed limit.
If the desire is to know exactly how fast you are going, then you have problems. Even if you get it exactly right for any of the stock size tires (or any other for that matter). Between new and legally worn out, the tire diameter will decrease by .5" (approximately 2%). How often are should someone replace their tires, or get the speedo recalibrated?
Scott
90SM
Alliance Member
Joined: Apr 2003
Posts: 4,101
Likes: 230
From: York, Pennsylvania
I find it interesting that although speedometers usually over-read actual
speed by several percent, odometers are usually surprisingly accurate
(at least on all the cars I've checked it on). And I always calculate gas
milage by miles driven over gallons used per tankful.
speed by several percent, odometers are usually surprisingly accurate
(at least on all the cars I've checked it on). And I always calculate gas
milage by miles driven over gallons used per tankful.
5th Gear
Joined: Nov 2003
Posts: 628
Likes: 0
From: Indianapolis
(replying to Misfitoy) I have my Scangage II hooked up all the time for the same reason (plus the other info displayed). In addition to the built in difference between the OBD II vehicle speed reading and what gets sent to the speedometer, I haven't had stock diameter tires since October 2002. The Scangage lets you account for the tire diameter difference in the value it displays, in 1% increments. It matches up quite well with the GPS readings, on the occasions that I've had a borrowed GPS unit in the car.
Scott
90SM
Scott
90SM
OVERDRIVE
Joined: May 2003
Posts: 7,926
Likes: 41
From: Greensboro, NC
Just out of curiousity, is the concern over speedometer accuracy based on fuel economy concerns, the desire to avoid breaking speed limits, or just the desire to know exactly how fast you are going?
The thread seems to have wandered over towards the fuel economy side, in which case I would argue that the speedometer accuracy is far less important that other factors.
If the concern is to avoid breaking speed limits, use any of the stock size tires and you won't get a ticket if the speedomenter reading matches the posted speed limit.
If the desire is to know exactly how fast you are going, then you have problems. Even if you get it exactly right for any of the stock size tires (or any other for that matter). Between new and legally worn out, the tire diameter will decrease by .5" (approximately 2%). How often are should someone replace their tires, or get the speedo recalibrated?
Scott
90SM
The thread seems to have wandered over towards the fuel economy side, in which case I would argue that the speedometer accuracy is far less important that other factors.
If the concern is to avoid breaking speed limits, use any of the stock size tires and you won't get a ticket if the speedomenter reading matches the posted speed limit.
If the desire is to know exactly how fast you are going, then you have problems. Even if you get it exactly right for any of the stock size tires (or any other for that matter). Between new and legally worn out, the tire diameter will decrease by .5" (approximately 2%). How often are should someone replace their tires, or get the speedo recalibrated?
Scott
90SM
It turns out that that was probably a "typo" on someone's part so the actual difference in OE 16" and 17" is probably only a few 10ths of an inch.
At this point I think the idea is to see if we can somehow turn this relatively uninteresting thread into the longest thread ever (longshot) by paying meticulous attention to each other's posts.
Considering the amount of time spent discussing tiny distances in the lowering spring threads, I think our discussion of a 1" difference in tire diameter (which will lower your car by .5" by the way) is merited.
5th Gear
Joined: Nov 2003
Posts: 628
Likes: 0
From: Indianapolis
My original concern was a 1" difference in wheel/tire diameter between OE 16" and 17" offerings published in Tire Rack - which is a lot in what shoud be a "precision" automobile.
It turns out that that was probably a "typo" on someone's part so the actual difference in OE 16" and 17" is probably only a few 10ths of an inch.
At this point I think the idea is to see if we can somehow turn this relatively uninteresting thread into the longest thread ever (longshot) by paying meticulous attention to each other's posts.
Considering the amount of time spent discussing tiny distances in the lowering spring threads, I think our discussion of a 1" difference in tire diameter (which will lower your car by .5" by the way) is merited.
It turns out that that was probably a "typo" on someone's part so the actual difference in OE 16" and 17" is probably only a few 10ths of an inch.
At this point I think the idea is to see if we can somehow turn this relatively uninteresting thread into the longest thread ever (longshot) by paying meticulous attention to each other's posts.
Considering the amount of time spent discussing tiny distances in the lowering spring threads, I think our discussion of a 1" difference in tire diameter (which will lower your car by .5" by the way) is merited.
Scott
90SM
I think we can assume a linear relationship between number of revolutions of the wheel and gas consumption: 1 revolution = X consumption; 2 revolutions = 2X consumption. therefore 4% change in diameter = 4% change in revolutions = 4% change in gas consumption (ignoring K constant factors such as idling, exponential efect of drag as speed increases above 50 mph, etc.)
obviously we can make the 4% back by avoiding left turns where possible (as UPS has done for a multi-million dollar saving on gas).
does anyone think my comment is too geeky? please let me know.
obviously we can make the 4% back by avoiding left turns where possible (as UPS has done for a multi-million dollar saving on gas).
does anyone think my comment is too geeky? please let me know.
between RPM and wheels? lol how about that huge cheeze burger
you just ate... where does that go into the equation? oh, and that
nice large drink... (and dont forget the cheeze cake). lol
5th Gear
Joined: Oct 2002
Posts: 779
Likes: 4
This had been discussed at length in many forums. GPS's are a great way to calibrate your speedometer.
- Mark
5th Gear
Joined: Oct 2002
Posts: 779
Likes: 4
Another aspect of this. Keep in mind that mfgs are under the certification standards of many different countries for speedo and odo accuracy.
Typical required speedo accuracy for certification is that the speedo can read quite a bit high but NEVER read too low (e.g., -0%, +10%). Thus, to take care of all the factors that might cause a speedo to read too low, the mfg has to delberately bias the speedo high. This is done in the interest of safety.
And odo's are certified to a different standard. Typically this is a symmetric standard (e.g., +/- 2%). Here, the regulators are trying to avoid mfgs deliberately having odometers read high which would cause emissions warranties to expire prematurely.
Combine these two requirements and you have speedos that tend to read high and odometers that tend to read more/less accurately, even though they're both working off the same data. IOW, they are deliberately inconsistent.
Again, you can thank the bureacrats for all of this, looking out for your safety and economic interests.
- Mark
Typical required speedo accuracy for certification is that the speedo can read quite a bit high but NEVER read too low (e.g., -0%, +10%). Thus, to take care of all the factors that might cause a speedo to read too low, the mfg has to delberately bias the speedo high. This is done in the interest of safety.
And odo's are certified to a different standard. Typically this is a symmetric standard (e.g., +/- 2%). Here, the regulators are trying to avoid mfgs deliberately having odometers read high which would cause emissions warranties to expire prematurely.
Combine these two requirements and you have speedos that tend to read high and odometers that tend to read more/less accurately, even though they're both working off the same data. IOW, they are deliberately inconsistent.
Again, you can thank the bureacrats for all of this, looking out for your safety and economic interests.
- Mark
OVERDRIVE
Joined: May 2003
Posts: 7,926
Likes: 41
From: Greensboro, NC
Total BS. At reasonbly steady state speeds, any consumer GPS sold today is highly accurate (and precise) in measuring speeds. The errors in absolute position are not significant because they tend to be the same from measurement to measurement and speed is computed based on the change in position, not the absolute position.
This had been discussed at length in many forums. GPS's are a great way to calibrate your speedometer.
- Mark
This had been discussed at length in many forums. GPS's are a great way to calibrate your speedometer.
- Mark
OVERDRIVE
Joined: Aug 2006
Posts: 7,201
Likes: 8
From: Norfolk, VA
Ditto - the real-world measurements for GPS speed accuracy show that while you won't always get the +/- 0.1 MPH that Garmin and the other manufacturers advertise, the accuracy is virtually always within 0.8 MPH of your actual speed.
This is assuming of course that you're not trying to use the GPS to measure your speed on a skidpad or an autocross course. If it takes you ten seconds to go around a skidpad, and your GPS only samples your position once per second, it will convert your car's circular path around the pad into a ten-sided polygon, and will underestimate your speed as a result.
This is assuming of course that you're not trying to use the GPS to measure your speed on a skidpad or an autocross course. If it takes you ten seconds to go around a skidpad, and your GPS only samples your position once per second, it will convert your car's circular path around the pad into a ten-sided polygon, and will underestimate your speed as a result.
OVERDRIVE
Joined: May 2003
Posts: 7,926
Likes: 41
From: Greensboro, NC
Ditto - the real-world measurements for GPS speed accuracy show that while you won't always get the +/- 0.1 MPH that Garmin and the other manufacturers advertise, the accuracy is virtually always within 0.8 MPH of your actual speed.
This is assuming of course that you're not trying to use the GPS to measure your speed on a skidpad or an autocross course. If it takes you ten seconds to go around a skidpad, and your GPS only samples your position once per second, it will convert your car's circular path around the pad into a ten-sided polygon, and will underestimate your speed as a result.
This is assuming of course that you're not trying to use the GPS to measure your speed on a skidpad or an autocross course. If it takes you ten seconds to go around a skidpad, and your GPS only samples your position once per second, it will convert your car's circular path around the pad into a ten-sided polygon, and will underestimate your speed as a result.
Originally Posted by G2Xtreme
What is the accuracy of the G2X?
Relative Accuracy of GPS Data
When working with GPS signals, it is difficult to quote absolute accuracy, due to a variety of factors. The number of satellites in view, the time of day, the location of the G2X GPS antenna along with natural and man made obstacles can affect accuracy.
However, when using GPS signals for track mapping, lap / segment timing and speed, the G2X and Datalink II software provide exceptionally accurate data, which has been tested and verified in years of on track usage.
Speed
In typical usage, the speed accuracy is approximately .10 MPH / .16 KMH.
Laptime
In multi-year testing of G2X vs. trackside beacon laptimes, a variance of .05 has been noted. GPS based laptimes are dependant only upon vehicle location, and can not be affected by sunlight, signal blockage from other vehicles, power loss to the transmitter, or other problems associated with trackside transmitters.
Track mapping
G2X GPS track maps are created from plotted vehicle location, thus eliminating positional errors that are inherent to inertia based systems. G2X GPS track mapping is capable of easily producing accurate track maps from courses that combine highly bank and extremely flat areas (such as the road course at Daytona), along with standard closed course race tracks. In addition, the G2X can create open course track maps, such as would be used in auto cross, hill climbing and other events where the start and finish are located at two different locations.
Relative Accuracy of GPS Data
When working with GPS signals, it is difficult to quote absolute accuracy, due to a variety of factors. The number of satellites in view, the time of day, the location of the G2X GPS antenna along with natural and man made obstacles can affect accuracy.
However, when using GPS signals for track mapping, lap / segment timing and speed, the G2X and Datalink II software provide exceptionally accurate data, which has been tested and verified in years of on track usage.
Speed
In typical usage, the speed accuracy is approximately .10 MPH / .16 KMH.
Laptime
In multi-year testing of G2X vs. trackside beacon laptimes, a variance of .05 has been noted. GPS based laptimes are dependant only upon vehicle location, and can not be affected by sunlight, signal blockage from other vehicles, power loss to the transmitter, or other problems associated with trackside transmitters.
Track mapping
G2X GPS track maps are created from plotted vehicle location, thus eliminating positional errors that are inherent to inertia based systems. G2X GPS track mapping is capable of easily producing accurate track maps from courses that combine highly bank and extremely flat areas (such as the road course at Daytona), along with standard closed course race tracks. In addition, the G2X can create open course track maps, such as would be used in auto cross, hill climbing and other events where the start and finish are located at two different locations.
Last edited by MSFITOY; Oct 26, 2007 at 11:09 AM.
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