speedoguy

well your speedometer will never be 100% accurate

the closest my shop can calibrate to is 2%


there are too many variables to guarantee 100% accuracy

but the only way to really tell how accurate your speedometer is, is to do a pulse count, and find out how many pulses per mile your vehicle speed sensor is outputting, and convert that to frequency at 60mph

for example, If i received a pulse count of 8,000 per mile, you would x that by 60 ( mph ) and divide by 3600 ( seconds in an hour )and i would have 133.33hz so I would inject 133.33 hz at the vss connector if your speedometer was accurate your gauge would show 60 mph, and odometer read 1 mile per minute


if both the odometer and speedometer had the same error ( lets say 10% slow ) I would know that I need to find a way to increase the number to pulses the vss output by 10% ( there are many devices that do this, the total repair usually runs around $300.00 )

If the speedometer was accurate, but the odometer was off by 3%, then the gauge itself s bad, and would need to be replaced

WellesleyScion

^^^ thanks for the info

yotalifer

i always understood that the latest gps devices are off by 50meeters or so to prevent a n accurate terrorist attack. with that the earlier ones 5 or so years old were more accurate than the ones we have today on the market.

mcbrew

Maybe that's the case with some cars, but not all. German cars all read about 4km/h high (about 2.5 MPH). This is because they have strict rules about cars not reading too low of a speed. The computer knows the true speed, but outputs a slightly higher speed to the speedometer. You can actually read both data streams side by side. The odometer, however, reads the true mileage (as close as the true vehicle speed is calibrated). Many modern cars also have "dummy" temperature gauges that read a smoothed out temperature to keep people from freaking out if the temp rises as they are cruising up a mountain at 80 MPH. Again, the computer knows the true temperature... it just outputs a different reading to the gauge.

My xD reads consistently 1 MPH low at 60 MPH. In other words, I am going 61 when the needle is on 60. I have tested that with two GPS devices -- a handheld and a windshield-mount nav system.

There used to be an error introduced into the GPS signals, which was called Selective Availability. It was for essentially the reason you stated -- to keep civilians from using GPS for weapons guidance. However, Selective Availability was shut off in 2000. We have the FAA to thank for that, since they pressured the US government to turn off SA in order to save millions of dollars in maintaining their own corrective signals.

There are errors in the GPS signal, but they are mainly due to transmissions being disturbed by the ionosphere as well as atomic clock drift. However, many modern GPS units have accuracy of up to 1 meter laterally and 1.5 meters vertically with WAAS enabled. WAAS is a series of ground-based stations that read GPS signals and calculate the error. The correction factor is then transmitted to WAAS satellites and can be picked up by many GPS devices (both of mine). The correction factor is transmitted about ever five seconds. This allows for very accurate positioning.

The

mcbrew

Okay, so I have a little bit of an update:

I got my handheld GPS (WAAS enabled) out and set it on the dash next to the nav unit. I took a little drive and this is what I came up with:

Trip meter on the xD = 8.3 miles (it had JUST turned over to 8.3 as I pulled in the driveway)
Trip meter on the nav unit = 8.6 miles
Trip meter on the handheld unit = 8.58 miles

So, since the handheld has better precision, that's a difference of 0.28 miles, which puts the xD's odometer at about 3.37% off (reading low). That pretty much corroborates what I saw with the previous test.

trikkonceptz

Now I was unaware of this glitch with the speedo .. but after reading through all this, I may have the low tech fix for you all ....

When I changed my gauge face I had to pull my needle to do it, well if you pull your needle and turn your key to on and set it back on 0, you should be fine. Right now my speedo is accurate with my scangauge and with any and all those radar station devices you see on the hwy.

So hope this helps ...

mcbrew

The speedo is pretty accurate... only reads about 1 MPH low. The odometer is off by more than 3%. The speedo and odometer do not jive with each other. Same thing with my Golf, but the other way around -- the odometer was just about spot on and the speedo read about 2.5 MPH high. All modern German cars do.

biancoxb

Its pretty funny that I read this but every time when I pass one of those how fast you are going signs too, it always reads a couple of miles under, strange . .

mcbrew

Not unusual. That type of radar is only accurate head on (moving directly towards or away from the radar gun). Since they are on the side of the road, cars are almost always moving at somewhat of an angle to them. That's usually why they read too low.