DIY Speedo Calibration

[Lightflyer1]

IIRC airbags need VCDS or dealer.

 

[B1nky]

I just want to say this, because apparently everyone thinks the opposite.

Never consider your GPS speed to be absolutely accurate. It isn't. The US government introduces a bit of error into the GPS system for civilians. This is so a civilian can't use GPS to direct a weapon at a specific target. The error is around 10 meters (notice next time you exit how your GPS might not recognize immediately that you left the highway), which is more than accurate enough for most things, but since the error is randomized, you can't count on your GPS calculating your speed accurately.

If your speedo says 70 MPH and your GPS says 75 MPH, you're probably going 71-72 MPH. If your speedo says 70 MPH and your GPS says 72 MPH, you're probably going 70 MPH.

If you can't use GPS to set your speedo correctly, what can you use? Don't use radar, because it's also prone to error (and it gets exponentially worse the further you get away from the radar beam, since the beam spreads.. a radar beam is only good for around 4 lanes of traffic, and after that it's the size of a car or two and not accurate). I think that a dyno might be able to measure the speed your wheels are actually traveling, and THAT is what I'd use to tune my speedo. If that's not an option, leave it alone.

While I agree and understand the restrictions placed on a GPS by the feds. I find it funny that the only car I own that does not agree with the GPS is the "German Engineered" car built in Mexico. Not one of the two Japanese Engineered cars built in the USA or the American Engineered car built in the USA and Canada. The discrepancy has been verified via pacing with these and other cars.
The helpful service persons at my local VW dealerships; two different ones both said the same thing. "Five miles an hour at 60 mph is within the limits set by the government. Nothing we can do about it." That was while it was under warrantee. I wonder how much they would charge to fix it now. Speed-o-meter says I am going 65 when I am doing 60. Not happy!!!

 

[D-Cell_Mekanick]

While I agree and understand the restrictions placed on a GPS by the feds. I find it funny that the only car I own that does not agree with the GPS is the "German Engineered" car built in Mexico. Not one of the two Japanese Engineered cars built in the USA or the American Engineered car built in the USA and Canada. The discrepancy has been verified via pacing with these and other cars.
The helpful service persons at my local VW dealerships; two different ones both said the same thing. "Five miles an hour at 60 mph is within the limits set by the government. Nothing we can do about it." That was while it was under warrantee. I wonder how much they would charge to fix it now. Speed-o-meter says I am going 65 when I am doing 60. Not happy!!!

The first dealer I asked to correct my mph discrepancy told me the Feds have a 10% variance to be legal. I asked them to print out their paperwork Incase I was ever pulled over. I was told NO and that they always are 10% under rather than over. Now I don't know if that is true or not, but mine reads about 3 mph faster than actual mph.

 

[Dodoma]

How come I cannot see the pictures. Put the pictures so that members can see those.

 

[jackbombay]

I just want to say this, because apparently everyone thinks the opposite.

Never consider your GPS speed to be absolutely accurate. It isn't. The US government introduces a bit of error into the GPS system for civilians. This is so a civilian can't use GPS to direct a weapon at a specific target. The error is around 10 meters (notice next time you exit how your GPS might not recognize immediately that you left the highway), which is more than accurate enough for most things, but since the error is randomized, you can't count on your GPS calculating your speed accurately.

If your speedo says 70 MPH and your GPS says 75 MPH, you're probably going 71-72 MPH. If your speedo says 70 MPH and your GPS says 72 MPH, you're probably going 70 MPH.

If you can't use GPS to set your speedo correctly, what can you use? Don't use radar, because it's also prone to error (and it gets exponentially worse the further you get away from the radar beam, since the beam spreads.. a radar beam is only good for around 4 lanes of traffic, and after that it's the size of a car or two and not accurate). I think that a dyno might be able to measure the speed your wheels are actually traveling, and THAT is what I'd use to tune my speedo. If that's not an option, leave it alone.

GPS is only "inaccurate" for location, not speed AFAIK.

It is extremely simple to check the GPS though, set the cruise at 60 MPH on a highway and start a stopwatch when you cross a mile marker, drive 15 miles and if it took 15 min to drive 15 miles you were going 60 MPH.

 

[ZiggyTheHamster]

GPS is only "inaccurate" for location, not speed AFAIK.
It is extremely simple to check the GPS though, set the cruise at 60 MPH on a highway and start a stopwatch when you cross a mile marker, drive 15 miles and if it took 15 min to drive 15 miles you were going 60 MPH.

Also, I've since discovered that the intentional civilian GPS inaccuracy has been removed. So it should be a good indicator of speed.

 

[Mike in Anchorage]

GPS is only "inaccurate" for location, not speed AFAIK.

It is extremely simple to check the GPS though, set the cruise at 60 MPH on a highway and start a stopwatch when you cross a mile marker, drive 15 miles and if it took 15 min to drive 15 miles you were going 60 MPH.

If GPS is inaccurate for location, the way to find speed is also off, since it uses the difference in location divided by the difference in time to find the average speed.
Also, tracking for 15 minutes at 60 mph will test your speedometer and your cruise control, but you will not know which variance is due to which system.
Life cane be tough. Just be glad you're not traveling at significantly relativistic speeds.

 

[jackbombay]

If GPS is inaccurate for location, the way to find speed is also off, since it uses the difference in location divided by the difference in time to find the average speed.

Also, tracking for 15 minutes at 60 mph will test your speedometer and your cruise control, but you will not know which variance is due to which system.
Life cane be tough. Just be glad you're not traveling at significantly relativistic speeds.

I can speak to the accuracy of the Mk4 cruise control, it is very good, many smart phone apps can log speed vs time on a graph and the line is amazingly straight.

But maybe you're right, that there is no way that any of us can determine if the speedo in our cars is accurate or not...

 

[slidewayz]

I did the dash mod just now.
My speedo used to read about 7kmh higher than actual at highway speeds.

I heard a pop sound when initially moving it gently counter clockwise.

Put it all together and now it reads 3kmh lower than actual speed @ 100kmh. Its 5kmh low at 60kmh, 4kmh low at 80kmh.

I tried 4 times to adjust it with nothing changing - whenever I turn it clockwise, it won't hold that position by the time the cluster is installed - it always goes back to a hard stop the equivalent of about 7kmh below 0.

y2k, or if anyone else knows how to properly re-adjust it forward and have it 'stick', I'd like to know. PS - I don't use GPS - I use an ELM327 bluetooth adapter and the program 'Torque' on my android phone which I've previously verified with GPS.

The funny thing is - the car KNOWS the correct speed - its just the needle that is out, not the software or vag.com stuff. If you were to change settings with vag.com, you would then mess up your odometer.

Thanks

 

[Hondroid]

After all these many pages on this thread, has anyone tried moving or removing the speedo needle on a Golf TDI '10-'11. Tried all VCDS settings with no change. I am interested if the needle will come off easily.

 

[BRMTDI]

There is a way to do this completely through VCDS alone, have to find the link to it.