Speedometer not accurate?
06-20-2008, 01:32 PM
#12
Platinum Member
Join Date: 05-09-2006
Location: "Upland" Mesa, Arizona
Posts: 6,805
2 "things".....
You are taking the position that the speedometer needle is off. Maybe it's not, and the crusie control is 1-2 mph off (which, in an economy car, is not inusual).
Check the speedo with the GPS unit as you suggested. Then let us know what the result is. Most people have found their speedometers are dead-on (I checked mine with 2 GPS units in tandum....was dead nuts on).
You can also check you speedometer by the mile post marker method. Use an Interstate, US or State highway with the mile markers. Do 60 MPH, indicated, and set the cruise control. When you reach the first mile marker, start a stop watch (or the sweep second hand on a wrist watch) and stop it at the next mile marker. It should read 60 seconds or 1 minute.
You are taking the position that the speedometer needle is off. Maybe it's not, and the crusie control is 1-2 mph off (which, in an economy car, is not inusual).
Check the speedo with the GPS unit as you suggested. Then let us know what the result is. Most people have found their speedometers are dead-on (I checked mine with 2 GPS units in tandum....was dead nuts on).
You can also check you speedometer by the mile post marker method. Use an Interstate, US or State highway with the mile markers. Do 60 MPH, indicated, and set the cruise control. When you reach the first mile marker, start a stop watch (or the sweep second hand on a wrist watch) and stop it at the next mile marker. It should read 60 seconds or 1 minute.
06-20-2008, 03:12 PM
#13
Senior Member
Join Date: 10-19-2005
Location: Miami
Posts: 20,687
You can also check you speedometer by the mile post marker method. Use an Interstate, US or State highway with the mile markers. Do 60 MPH, indicated, and set the cruise control. When you reach the first mile marker, start a stop watch (or the sweep second hand on a wrist watch) and stop it at the next mile marker. It should read 60 seconds or 1 minute.
First.......Make sure you have someone not driving to do the timing
Second......Just how accurate do you think this method really is w/o having timing lights to ensure that you start/stop measuring at the exact same point?
06-20-2008, 03:16 PM
#14
Senior Member
Join Date: 10-19-2005
Location: Miami
Posts: 20,687
Just ran a few errands at noticed while at a stop light, that from my vantage point the needle was sitting just above 0. I will have to check to see how it reads on the next interstate trip to see how accurate the needle is in the "12 o'clock position" from the same vantage point.
06-20-2008, 05:26 PM
#15
Platinum Member
Join Date: 05-09-2006
Location: "Upland" Mesa, Arizona
Posts: 6,805
. But when I was making frequent trips (every weekend) to SoCal, I utilized this on Interstate 8, after assuring that I would not interfer with other traffic. My measurements were only off by 2 tenths of a second. Even if your reaction time is really off, 2 seconds for example, it's only 3 foot discrepancy. Which is an insignificant amount. I ALWAYS wear one of many chronograph watches, so that part of accuracy is not a problem. This is the same method police departments use for school zones........hway patrol uses from aircraft. Courts have uphelp the use as accurate and reasonable.......basic math...time and distance.
I also have seen several roads throughout the US that are specifically marked for the speedometer test, by their respective road commisions.
Bad mistake in my Math
....none of the 200+ readers are going to correct it? Looks like I ran a good bluff !!
Last edited by Snoopy; 06-21-2008 at 01:15 AM.
06-22-2008, 08:46 AM
#17
Senior Member
Join Date: 08-18-2006
Location: Hatboro, PA
Posts: 1,036
I saw it also. Just didn't reply.
Reason: I doubt you would be off by a full second unless you weren't paying attention when you passed the mark.
And... being off by a full second, would only make your reading be off by about ± 1 mph. Pretty tough to see that on an analog gauge (while going 60mph). 
Reason: I doubt you would be off by a full second unless you weren't paying attention when you passed the mark.
And... being off by a full second, would only make your reading be off by about ± 1 mph. Pretty tough to see that on an analog gauge (while going 60mph).
06-23-2008, 11:21 AM
#20
Senior Member
Join Date: 03-31-2008
Location: CLEVELAND
Posts: 772
I've notice the same on mine. Is 1 or 2 mph really worth bothering the dealer with? Is it really worth being HHRless for 2 or more days?
I look at it this way..........
If the computer is truly accurate vs the needle, then there is a built in safety margin if you are traveling the speed limit (yea right, like anyone does that ). Just last month my wife got cited for doing 4 miles over the posted speed limit in a "prestigous" part of town were the local PD is very strict. Maybe if she had that built in margin...........
I look at it this way..........
If the computer is truly accurate vs the needle, then there is a built in safety margin if you are traveling the speed limit (yea right, like anyone does that
). Just last month my wife got cited for doing 4 miles over the posted speed limit in a "prestigous" part of town were the local PD is very strict. Maybe if she had that built in margin...........