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Old 07-11-2003, 10:36 AM   #1
Mister Toro
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Post Speedo Analysis

Just wanted to share my findings about a speedometer error on my new STi. During the 'getting to know you' period between me and any new car that I plan to live with for a good long time, I usually do some measurements and calibrations. One aspect is understanding the inevitable errors in the guages. So using a GPS empowered laptop, I plotted the speed that my speedo read vs. 'actual speed' as measured by the GPS. Even though there can be errors in GPS velocity measurements, I find that when done with open sky on straight level roads, and averaged over a reasonably long period of say, 30 seconds, one can arrive a very reliable, repeatable results. Here is what I found:



So this translates to roughly a -1mph offset error with a -6% linearity error. These results were measured with 36/32 psi F/R in the stock tires and with ~700 miles on the car. As the tires wear, this error will only grow. I realize that every car will be different, but I wanted to make my own data available. Can anyone else make similar measurements with theirs?
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Last edited by Mister Toro; 07-11-2003 at 12:28 PM.
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Old 07-11-2003, 10:43 AM   #2
Guyver
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I can confirm this. I stared a thread about this error in May.
http://forums.nasioc.com/forums/show...ht=Speedometer

Here Are my Results

Displayed Speed Vrs Actual

35 MPH = 32 Mph

45 Mph = 41.8 Mph

55 Mph = 51 Mph

65 Mph = 61.5 Mph

70 Mph = 66 Mph

Tested with Garmin GPS Emap and a Friendly Police officers radar gun.


To cruise with traffic at 70 Mph I will need to go a displayed 75 mph.

Just think of the lost warantee miles.
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Last edited by Guyver; 07-11-2003 at 10:49 AM.
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Old 07-11-2003, 12:24 PM   #3
Mister Toro
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Quote:
Originally posted by Guyver
I stared a thread about this error in May...

Just think of the lost warantee miles.
Guyver
Oops, I must have missed that thread in my searching. I'll be sure to check the odometer as well, but I have a feeling that the error might indeed be only in the speedometer.

I do know that in Europe, there are some pretty tough laws against building a car that has a speedo error in the other direction (car goes faster that it says), so almost all Eurocars have this sort a error. But 6% is just too steep. If it were up to me, I would prefer proper calibration; I wonder what is involved here...
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Old 07-11-2003, 02:30 PM   #4
STIffy
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I would definitely check the odometer over an extended period on the highway using mile markers. I have not checked the accuracy of my speedo yet, but I work at a speedometer repair shop and I can tell you that most new cars have a speedo error that tends to make them read 1-3miles fast. I assume this is partially to avoid possible lawsuits. Most of the cars we check have accurate odometers, it is just that the needle is placed on to the stepper motor shaft a miles ahead of where it should be.


George
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Old 07-11-2003, 03:01 PM   #5
nt5k
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There was a CD article on this a long time ago. They pretty much found that most speedos were off but odos were accurate.
I wouldn't get bent out of shape about it..
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Old 07-11-2003, 04:22 PM   #6
inchoate
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Quote:
Can anyone else make similar measurements with theirs?
my own gps results are very close to yours, i don't have a pretty visual aid though. not too big a deal imho, but it's worth knowing how far off the speedo is.
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Old 07-11-2003, 04:40 PM   #7
dsmperformance
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Default Re: Speedo Analysis

Quote:
Originally posted by Mister Toro
Can anyone else make similar measurements with theirs?
Mine is close to your results also.
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Old 07-11-2003, 04:45 PM   #8
atlien
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My S.E.C.S. Monitor always show about 3mph lower than the speedo no matter what speed I am going. Seems to be like what was said before, that the needle is off a little on the stepper motor shaft
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Old 07-11-2003, 05:03 PM   #9
metoo
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I KNEW I WASN"T CRAZY
I knew something had to be wrong. When the Speedometer was reading 80, it didn't feel or seem like I was going over 75. My butt dyno usually measures speed pretty well.
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Old 07-11-2003, 06:55 PM   #10
Jeff02WRX
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After the mistake of putting white face guages in when I switched back to stock I put the needle a little too high, I figured better higher then lower. I drove it with my radar gun and it reads about 6-7 mph high. Helps to keep me out of trouble I guess.
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Old 07-12-2003, 12:43 AM   #11
andys_911
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I was having the same experience. I couldn't figure out why all the old ladies wanted to race me when I was being good and driving the speed limit... I mean I was in the way!

I checked my speedo against a GPS unit and also double checked with a stop watch over a 15 mile section of fairly flat straight freeway. To get 75 mph actual speed I needed to set the cruise so the speedometer read 79 mph. My GPS only reads whole numbers, but my average speed was within .2 mph of 75 actual measured via the stopwatch over the 15 miles. I did this twice with the same results. I also checked the odo and it seems to be accurate to within .1 mile over the 15 mile test section.

I also did some quick checks of speedo vs. GPS at lower speeds and it seems to be about 3 mph off at 40.

So my speedo is about 5% off at 75 mph, which I believe is within the legal requirement for accuracy. There's a federal standard that I haven't had any success finding... FMVSS xxx something.

Just for grins I checked my 2003 F-150 the following day. Dead accurate! No reason I can think of that this couldn't be fixed with a new better calibration. If Ford can do it, so could Subaru!
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Old 07-12-2003, 09:04 AM   #12
robmarch
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is there a tire size that would fix the problem on the 17" rims and still not rub, etc?

so, the guy in the top speed thread who said he hit 175 is actually going 175 -1 - 10.5 = 163.5. That makes a lot of sense.
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Old 07-12-2003, 09:44 AM   #13
ha-evolution
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It's a built in safety feature from Subaru called moving violations reduction speedometer calibration. I don't think it's going to help me though, a 5mph reduction from what the speedo reads is miniscule since it so easy to be going 30 or 40 over the posted speed limit.
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Old 07-12-2003, 11:49 PM   #14
ldivinag
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my experience:

using a garmin 12xl unit, i saw a +3.4-3.6 diff between actual speed and speedo readout.

one thing though, the ODO is NOT affected by this.

i did a 50 and 50 mile drive and at 50 miles, i didnt get a noticeable diff. at 150 i saw a .2

while that is not big, over the life of the car, the warranty mileage MIGHT be affected.

also, looking at the factory manuals, the odo and the speedo is driven via the same system, but splits off an the speedo has another that it goes through.

in these threads:

http://www.imprezawrxsti.com/postnuk...75567b99cb2522
http://www.imprezawrxsti.com/postnuk...75567b99cb2522

you can see my documentations...
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Old 07-13-2003, 02:33 AM   #15
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I had noticed this when we were on the highway. I checked the GPS tonight, and I got approximately, 35mph speedo = 33, 45 = 42, 55 = 51.5, 65 = 61. So, like others, my speedometer is reading about 3 to 4 mph fast. I did also check with my PSI3 unit and its mph reading is within 1 mph of the GPS. So my guess is that the ECU is pretty accurate with mph and the error is in the speedometer display.

The good thing is that I can use my PSI3 and its top speed memory during autocrosses and hill climbs and trust its result without having to apply any corrections. The GPS is great but at about 1 reading per second it is perfect for highway cruising but not for the top speed while accelerating right before hard braking. My Geez unit is supposed to calculate mph, but it is way off in my experience, so hasn't been useful for seeing top speed (besides, I need to use the home computer to see the results, so it isn't useful for comparing two cars at an event anyway).
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Old 07-13-2003, 08:37 PM   #16
Mr. Grinch
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Why not also record tach readings and add the speed calculated from the tach to the graph? Just for kicks.

BMW's are also notoriously inaccurate. But if you use the trip computer to calculate avg speed, it's quite accurate.

My Honda is quite accurate as well.
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Old 07-14-2003, 05:22 AM   #17
HFTuRbo
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Quote:
It's a built in safety feature from Subaru called moving violations reduction speedometer calibration.
Whatever the case, it certainly kept me going on Saturday while others were being pulled-over. It's not off far enough if you're really soaring and not paying attention to the speedometer. The `02 WRX was indicating a higher speed than actual as well.
I kinda like it that way as long as the odometer is correct it's no biggie!
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Old 07-14-2003, 11:04 AM   #18
strangerq
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Speedometers are designed to read fast, because federal regulations specify that they not read slow (if they read fast they are still within spec.).....this is more for legal reasons than safety.

Stops you from blaming the manufacturer for speeding tickets.

This is very well known, and happens with Honda, BMW, Lexus, Porsche etc..

I think car and driver did a systematic study once and did not find a single car where the speedo wasn't at least a couple mph fast.

It is by design.
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