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Speedo over-reading? How do I check?

Discussion in 'Motors' started by SlyReaper, 15 May 2010.

  1. SlyReaper

    Soldato

    Joined: 26 Apr 2008

    Posts: 6,583

    Location: Bristol, Old Blighty

    My car is a 17 year old Vauxhall (old enough to drive itself? Who knows). Anyway I suspect the speedometer over-reads by a fairly hefty amount. This is because I can go past those 30 signs that light up if you're speeding at an indicated 35 and still not set them off. Not to mention people behind me getting very impatient with me if I obey the speed limit on my speedo. Is there any good way I can check how much it is over-reading, or even better, a way to recalibrate it?
     
  2. PMKeates

    Capodecina

    Joined: 16 Jul 2004

    Posts: 14,052

    Do you have a nav device that can display speed? If not, a phone with GPS capability? You might be able to get an application that will display your speed on that, and it will be reasonably accurate.
     
  3. Jez

    Caporegime

    Joined: 18 Oct 2002

    Posts: 32,741

    Borrow a portable sat nav such as a tomtom and read the speed off that to check it before you do anything else :)
     
  4. Dogbreath

    Capodecina

    Joined: 18 Oct 2002

    Posts: 16,657

    Location: Devon

    This is quite possible if it's running on lower profile tyres than it should or if it's had a replacement gearbox with a different final drive fitted etc. Calibrating it yourself isn't easy at all, but there are companies that do this.
     
  5. SlyReaper

    Soldato

    Joined: 26 Apr 2008

    Posts: 6,583

    Location: Bristol, Old Blighty

    Satnavs show your speed? I never knew that.

    Cool. But being a jaded friendless misanthrope, I have nobody to borrow one off.

    Edit: Dogbreath, this car has never had any modifications like that.
     
  6. thepeganator

    Wise Guy

    Joined: 14 Sep 2008

    Posts: 2,486

    Location: Portsmouth/Salisbury

    My Tomtom shows my speed, and tells me that my Seat Cordoba over estimates my speed by about 9mph at 80mph (actually about 71), down to about 5 or 6 mph when going 40ish.

    Do you not have a satnav that tells you speed?
     
  7. SlyReaper

    Soldato

    Joined: 26 Apr 2008

    Posts: 6,583

    Location: Bristol, Old Blighty

    I have never owned a satnav of any kind. I feel they ruin the fun of getting lost in new places. :D
     
  8. PMKeates

    Capodecina

    Joined: 16 Jul 2004

    Posts: 14,052

    Do you have an old bicycle speedometer? You could use that to measure it.
     
  9. Howard

    Capodecina

    Joined: 12 Jul 2007

    Posts: 16,175

    Location: South East

    What phone do you have?
     
  10. PhillyDee

    Capodecina

    Joined: 12 Feb 2007

    Posts: 14,169

    Location: South Shields

    How?

    Theres nothing to clip the magnetic bit onto . . . . or the other bit.
     
  11. thepeganator

    Wise Guy

    Joined: 14 Sep 2008

    Posts: 2,486

    Location: Portsmouth/Salisbury

    Go next to a lorry on the motorway?

    Then you'll be going 56mph.
     
  12. SlyReaper

    Soldato

    Joined: 26 Apr 2008

    Posts: 6,583

    Location: Bristol, Old Blighty

    Again, I wasn't even aware such things existed. This thread is starting to make me feel like a luddite.

    Well, I'm taking the car to a garage on Monday to have new alternator bushes fitted. Will they be able to do a speedometer check while I'm there?

    Oh and I already know the speedo over-reads by 10mph when I'm stationary because that's where the resting pin is. :p
     
  13. Howard

    Capodecina

    Joined: 12 Jul 2007

    Posts: 16,175

    Location: South East

     
  14. metamorphpwner

    Wise Guy

    Joined: 6 Sep 2008

    Posts: 2,111

    Location: Near Hull

    get next to any car on the motorway/ dual carriageway, open your window and ask them their speed and compare it to yours.
     
  15. SlyReaper

    Soldato

    Joined: 26 Apr 2008

    Posts: 6,583

    Location: Bristol, Old Blighty

    One without GPS.
     
  16. stockhausen

    Capodecina

    Joined: 30 Jul 2006

    Posts: 11,971

    Long piece of string and a stop-watch :confused:
     
  17. bledd

    Don

    Joined: 21 Oct 2002

    Posts: 46,801

    Location: Parts Unknown

    Borrow one, you must know someone with one
     
  18. thepharcyde

    Sgarrista

    Joined: 17 Mar 2004

    Posts: 8,258

    Just go through a 30mph at an indicated 50mph. You will then get a speeding ticket that I'm sure will show your calibrated speed :)

    HTH
     
  19. SlyReaper

    Soldato

    Joined: 26 Apr 2008

    Posts: 6,583

    Location: Bristol, Old Blighty

    Genius. :D
     
  20. lawz

    Gangster

    Joined: 3 Dec 2005

    Posts: 458

    use 10% overread as a rule of thumb?

    Beg or borrow a satnav ;)