Speed: Speedometer vs. AP v2

[Loyale93]

The AP v2 has the ability to read live data. the other day, my buddy in the car noticed that it was reading a speed about 3 mph slower than my dashboard factory speedo.

Is the speedo mechanical or is it reading from the computer like the AP is?
Is there a way to set it?
Would this have an effect on my odometer?

PS - I did a quick search, and couldn't find anything like this. If someone out there is smarter than I... Thanks!!

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[rkramer]

the speedo is electronic off the same signal. no you can't calibrate it, and it is more or less just a matter of the needle being off of where it should be, it doesn't affect your odometer. Chances are your overall speed is probably off by 1-3mph anyway, check it with a gps.

[pasths]

I've found the speedo to be off by 9%. So the faster you go the more off it is.

[Loyale93]

Thanks for the input gentlemen. It sounds like I need to have a buddy in the car and check it at all speeds. the 3 mph is at highway speeds (about 75-80). So that 9% sounds like something I can check pretty easily. If it is a consistent 3 mph off, it sounds like a little flick of the needle will fix my problem.

[pasths]

[quote=Loyale93;17734013]So that 9% sounds like something I can check pretty easily. QUOTE]

I checked it using one of those radar displays the police dept. puts out. Luckily it was at an intersection that had U turns on both sides I started at 20mph and kept going by it at 10 mph more intervals. For every extra 10mph it was about 1 mph more off.

[richy_21]

^ yea im trying to find one so i can check mine.

[Alan]

Use a GPS. Handheld ones are down to under $70 now, and used ones can be found for $25 or so. Great devices for lots of other things too..

And my GPS shows my speedometer to be within 2 mph from 30-80 so the speedometer in MY 06 is pretty accurate. To some extent it's luck of the draw as the speedometer accuracy depends on the hairspring that restrains it and the magnetic clutch that drives it - both of which are subject to manufacturing tolerances.

[rkramer]

Originally Posted by Alan

To some extent it's lck of the draw as the speedometer accuracy depends on the hairspring that restrains it and the magnetic clutch that drives it - both of which are subject to manufacturing tolerances.

and tire diameter, which probably has just as big of an effect. (wear and tire pressure in particular, assuming stock size tires) also, not all tires are the same size, they vary by brand. (same indicated size, different actual size)

[MorallyBass]

I'm digging up this old thread, but I have almost the exact same speedo error in my 2004 STi, checked via GPS. When my Speedo reeds 80mph, I'm actually doing 74mph. Very frustrating.

[rkramer]

Originally Posted by MorallyBass

I'm digging up this old thread, but I have almost the exact same speedo error in my 2004 STi, checked via GPS. When my Speedo reeds 80mph, I'm actually doing 74mph. Very frustrating.

that is ALOT! (7.5%) That will knock almost 5000 miles off your 60000 mile powertrain warranty.

[Alan]

BTW - I ran up to 137 using the Scanguage, speedo and GPS a couple of weeks ago - and all agreed within 2 mph at all speeds from 60 to 137. I was very pleased. This was on a 2006 WRX with somewhat overinflated tires.

[LastResort]

Originally Posted by rkramer

that is ALOT! (7.5%) That will knock almost 5000 miles off your 60000 mile powertrain warranty.

NO! It is not safe to assume that is correct, because you don't know that they are affected the same way. In fact, the odometer is not off by the same margin of error. Several people have established that the odometer is correct to a fraction of a percentage. (assuming stock diamater).

Pleas see more here:
http://forums.nasioc.com/forums/show....php?t=1219695
Specifically look for the description of why the two systems (speedometer and odometer) are independent, and the testing done by people.