Speedo accuracy
#1
07-13-2010, 10:22 AM
Speedo accuracy
I've recently noticed that the speedo of my 2000 XKR is showing my speed as being significantly slower than 2 different GPS's (my handheld garmin, and my Escort radar detector that has a built-in GPS). When I'm doing 65 according to my speedo, both GPS's say I'm actually around 70.
I'm running the stock 18" double-five wheels so its not a problem related to non-standard wheel size.
I'm guessing that the GPSs are more likely to be right than my speedo, although I'm prepared to be corrected. I'd like to be sure my speedo is showing my speed accurately, especially as I live in Phoenix which is speed camera central (although the good news is speed cameras on all AZ freeways are being permanently unplugged by law this month actually). I'm predicting a subsequent big increase of speed cops though.
Does anyone know if its possible to adjust the speedo? If so, how?
I'm running the stock 18" double-five wheels so its not a problem related to non-standard wheel size.
I'm guessing that the GPSs are more likely to be right than my speedo, although I'm prepared to be corrected. I'd like to be sure my speedo is showing my speed accurately, especially as I live in Phoenix which is speed camera central (although the good news is speed cameras on all AZ freeways are being permanently unplugged by law this month actually). I'm predicting a subsequent big increase of speed cops though.
Does anyone know if its possible to adjust the speedo? If so, how?
Last edited by JustNiz; 07-13-2010 at 10:29 AM.
#2
07-13-2010, 10:40 AM
Senior Member
I remember hearing a while ago that speedometers on cars were notoriously wrong -- but I think that was '70's and '80s vehicles. But being off that much is quite a bit with stock wheels and tires on a modern vehicle.
If 2 sources that are the same with each other have you wrong, then I'd be inclined to believe them. Also, my GPS and speedometer are exactly the same.
Hope that helps. That's pretty interesting about the speed cameras.
If 2 sources that are the same with each other have you wrong, then I'd be inclined to believe them. Also, my GPS and speedometer are exactly the same.
Hope that helps. That's pretty interesting about the speed cameras.
#3
07-13-2010, 10:45 AM
Veteran Member
You mention wheels, but not tires. Make sure you have the correct size tires on as well. [245/45ZR18 front, 255/45ZR18 rear]
As far as adjusting the speedo, I'm not sure on these cars, but quite a few have a gear that would need to be changed for a different sized one to recalibrate (I've seen this done when people switch wheels/tires)
As far as adjusting the speedo, I'm not sure on these cars, but quite a few have a gear that would need to be changed for a different sized one to recalibrate (I've seen this done when people switch wheels/tires)
#5
07-13-2010, 11:39 AM
#6
07-13-2010, 12:11 PM
Veteran Member
Because wide tires on the back look groovy. I have 285/30ZR19's on the rear, which makes each tire about an inch and a quarter wider than stock. Of course, they are also lower profile tires, but that means the sidewall on the 285's are about the same height as the sidewall on the 255's. That keeps my speedo accurate. Like h20boy, my speedo is accurate to within +1 mph of my GPS. Usually it is spot-on.
#7
07-13-2010, 12:22 PM
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#8
07-13-2010, 12:45 PM
Veteran Member
Sometimes I have these urges to lash out. I just have to wait a few minutes...
OK... The second number in tire size is the ratio of the sidewall to the tread width. So a 255/35/19 has a sidewall of 89 millimeters, or 3.7 inches. A 285/35/19 would have a sidewall of 100 millimeters, or 4.1 inches. That taller tire would cause the speedo to read slower than it's supposed to. To keep the speedo accurate I used a sidewall ratio of 30%. My tires are 285/30/19's not 285/35/19's. The sidewall on my wider tires is 85.5mm, or 3.5 inches.
Here's a good description of what the tire dimensions mean:
http://www.tirerack.com/tires/tirete....jsp?techid=7&
And here's a nifty tire size calculator. It shows you graphically how different tire sizes compare.
http://www.miata.net/garage/tirecalc.html
#10
07-13-2010, 01:04 PM
Veteran member
^ you'd have to find the flattest road, the mile markers would have be EXACTLY 1 mile apart, you'd have to start/stop your watch perfectly, keep a steady accelerator (cruise isn't THAT consistent)...man, so many variables to affect it. As more variables are in play, the more likely it won't be accurate.
But hey, its good enough for cops! (but easily refuted in court, as long as its close to the speed limit...its really only good for drastic speed offenses 20 mph over +
But hey, its good enough for cops! (but easily refuted in court, as long as its close to the speed limit...its really only good for drastic speed offenses 20 mph over +
#11
07-13-2010, 01:18 PM
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#13
07-13-2010, 02:12 PM
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#14
07-13-2010, 05:27 PM
Well, around my area, I can tell you how to know that you are going exactly 70. The vast majority of 18 wheelers drive at almost exactly 70, since they are all now equipped with GPS right back to their office. Is it just in my area, or do you guys also have some fools in those trucks driving along side by side at 70 mph for 5 or 10 miles, intentionally backing up the car traffic? (in what I suspect is some sort of revenge on us car drivers)
#15
07-13-2010, 06:15 PM
Because wide tires on the back look groovy. I have 285/30ZR19's on the rear, which makes each tire about an inch and a quarter wider than stock. Of course, they are also lower profile tires, but that means the sidewall on the 285's are about the same height as the sidewall on the 255's. That keeps my speedo accurate. Like h20boy, my speedo is accurate to within +1 mph of my GPS. Usually it is spot-on.
Fast Freddie
#16
07-13-2010, 06:27 PM
You mention wheels, but not tires. Make sure you have the correct size tires on as well. [245/45ZR18 front, 255/45ZR18 rear]
As far as adjusting the speedo, I'm not sure on these cars, but quite a few have a gear that would need to be changed for a different sized one to recalibrate (I've seen this done when people switch wheels/tires)
As far as adjusting the speedo, I'm not sure on these cars, but quite a few have a gear that would need to be changed for a different sized one to recalibrate (I've seen this done when people switch wheels/tires)
#17
07-13-2010, 08:28 PM
Veteran Member
This was a huge point of contention on the Z3 forums when I had my Z3. Apparently BMW intentionally makes their speedometers show a higher speed than you are actually travelling. Here's a copy and paste from the BWM equivalent of a TSB:
SUBJECT:
Maximum Permissible Speedometer Error
MODELS:
All
General Information:
A certain amount of "speedometer advance" is necessary to compensate for negative tolerances in tire diameter, electronic controls, and other factors. This advance reduces the risk of the speedometer displaying a speed that is less than the vehicle's actual speed.
The amount of speedometer advance can be calculated by inputting a frequency (using the DIS Tester, "Kombi Test Schedule") that corresponds to a given speed and noting the speed that is indicated by the speedometer. The maximum permitted speedometer advance is 10% of the actual (input) speed plus 2.4 mph.
Example:
Actual Speed = 50 mph
50 mph x 10% = 5 mph; 5 mph + 2.4 mph = 7.4 mph
Therefore, the permissible displayed speed is 50 to 57.4 mph.
(Note: The displayed speed must never be less than the actual speed).
No repairs should be attempted if the speedometer advance is within 10% + 2.4 mph.
NOTE: The amount of speedometer advance has no effect on recording of accumulated mileage in the odometer display. The odometer records total mileage digitally and does not incorporate any "advance" tolerances.
Warranty Status:
Information Only.
As I said above, I haven't found it to be an issue with the Jag, but in the BMW I didn't even use the speedometer, I just set my GPS on the speed display and used it. Maximum Permissible Speedometer Error
MODELS:
All
General Information:
A certain amount of "speedometer advance" is necessary to compensate for negative tolerances in tire diameter, electronic controls, and other factors. This advance reduces the risk of the speedometer displaying a speed that is less than the vehicle's actual speed.
The amount of speedometer advance can be calculated by inputting a frequency (using the DIS Tester, "Kombi Test Schedule") that corresponds to a given speed and noting the speed that is indicated by the speedometer. The maximum permitted speedometer advance is 10% of the actual (input) speed plus 2.4 mph.
Example:
Actual Speed = 50 mph
50 mph x 10% = 5 mph; 5 mph + 2.4 mph = 7.4 mph
Therefore, the permissible displayed speed is 50 to 57.4 mph.
(Note: The displayed speed must never be less than the actual speed).
No repairs should be attempted if the speedometer advance is within 10% + 2.4 mph.
NOTE: The amount of speedometer advance has no effect on recording of accumulated mileage in the odometer display. The odometer records total mileage digitally and does not incorporate any "advance" tolerances.
Warranty Status:
Information Only.
The speedometers on most cars these days get their signal from an electronic pulse out of the transmission. There are little widgets that you can buy that allow you calibrate the speedometer. They are put inline with the wire that delivers the pulses and they take the input pulses and then output the pulses either faster or slower depending on which way you need to adjust the speedometer.
#19
07-15-2010, 09:14 PM