Speedo (in)accuracy?
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This is a discussion on Speedo (in)accuracy? within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; Since I got my car, I've noticed that the speedo is off. It gets farther and farther off in a ...

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    Registered User teflon_jones's Avatar
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    Speedo (in)accuracy?

    Since I got my car, I've noticed that the speedo is off. It gets farther and farther off in a linear way the faster I go. When it reads 30, I'm doing 27. At 40, 36. When it says 50, 45. 60 is 54, 70 is 63. 80 is 72 and 90 is 81.

    I'm basing the actual figures on 1) those roadside speed monitoring signs and 2) speeds of other drivers on roads I frequently drive.

    Does anybody else have this problem, or is my car just a freak?
    Scott
    Past Subies - '11 STI Limited stage 1+ and '04 STi stage 2+
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    Would Never, Ever Say Something Bad About an Admin's Mom SonicWagon's Avatar
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    i once watched a 60 minutes where they found out that BMW speedos were off by 20%

    GPS is the only accurate measure that i know of.
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    Administrator Trainrex's Avatar
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    Do you have wheels other than stock size on the car?

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    Registered User jeffmm06's Avatar
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    Take the 22s off the car scott
    2005 WRB WRX "was" STAGE 2

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    Registered User teflon_jones's Avatar
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    Quote Originally Posted by Trainrex View Post
    Do you have wheels other than stock size on the car?
    Right now, yes. My winter set. But the problem is the same as with my summer wheels/tires which are both the stock parts. I've always found my speedo in every car I've owned to be off by a couple of MPH, but not in a linear way like this.

    Quote Originally Posted by jeffmm06 View Post
    Take the 22s off the car scott
    I refuse to take anything off my car that makes me a god to Honda owners.
    Scott
    Past Subies - '11 STI Limited stage 1+ and '04 STi stage 2+
    '13 Wrangler Rubicon - custom front bumper w/12k lb winch & off-road lights, skid plates, diff guards, etc.
    '04 Porsche 911 Turbo GT2 clone 600 HP/TQ
    '77 F-150 heavily modded for off-roading

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    He simply abides. SD_GR's Avatar
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    Quote Originally Posted by teflon_jones View Post
    Right now, yes. My winter set. But the problem is the same as with my summer wheels/tires which are both the stock parts. I've always found my speedo in every car I've owned to be off by a couple of MPH, but not in a linear way like this.
    Speedos are usually off a little and are usually optimistic when they're off, but it most often happens at higher speeds. You're seeing a consistent issue. Either the ratio has changed (wheels as mentioned) and it stays changed during both winter and summer, or (I'm just making a wild guess) maybe someone tried to do a lighting change on your instruments and put the needle back on offset? Does the needle zero correctly when the car is off? What happens with the needle sweeps at turn-on, does it go all the way across the gauge and back "normally?"

    I refuse to take anything off my car that makes me a god to Honda owners.
    My Honda rolls on only 16s.
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    Resident meany-rator 06wrx4me's Avatar
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    My speedo is dead on. and if there were any question of the gauge actually malfunctioning, my AP V2 MPH and my Speedo MPH are exactly the same, which also correspond to my buddies Garmin GPS speedo, and the road side signs.
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    Registered User teflon_jones's Avatar
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    Quote Originally Posted by SD_GR View Post
    If I was still under warranty I'd take it to the dealer. For it to be off by as much as it is, my tire diameter would have to be something like 1" less than the stock size. That would be a pretty good trick!
    My 24" tires make me a honda god or somethin...
    Last edited by 06wrx4me; 04-02-2008 at 08:16 AM. Reason: drr dumb moderator hit the wrong button sorry ;(
    Scott
    Past Subies - '11 STI Limited stage 1+ and '04 STi stage 2+
    '13 Wrangler Rubicon - custom front bumper w/12k lb winch & off-road lights, skid plates, diff guards, etc.
    '04 Porsche 911 Turbo GT2 clone 600 HP/TQ
    '77 F-150 heavily modded for off-roading

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    Registered User Deathrin's Avatar
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    I see this as a common problem in WRX you see the issue is acually burried deep in physics... with such a sweet ride and hard accel you are actually bending space and time around you!! The measurements you get from the road side radars and your car's speedo are based off two diffrent frames of ref. (Look into enstiens theory on relative motion) Have you notice your cars mass increasing as well....

    Sorry to much coffee on a slow wed at work
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    Resident meany-rator 06wrx4me's Avatar
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    Call cobb and see if they can make you a map or ask if you can go to a tuner and have them calibrate it for you. If the AP V2 can monitor MPH I don't see why it couldn't also calibrate the speedo.

    Also you could have a faulty or out of range speedo sensor on the side of the tranny. I'm sure there is a voltage range you can test it for. Being in a harsh environment that includes sand and salt, exposed or partially exposed electronics do not live very long. A simple "probe hole" in a wire is an access point for green rot. Giving that the speedo runs in mV, this could be your issue. Any type of corrosion would up the resistance and throw it off as well.

    So Inspect the wiring to the speedo sensor as well as the connector plug. Could be as simple as cleaning it with electronics cleaner and shooting it full of dielectric grease.
    2014 Ford Fiesta ST

    06 WRX, HAWKEYE # 001 <T-boned and sold>

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