speedo reads fast

Discussion in 'General Ranchero Help' started by neil brown, Jul 7, 2019.

  1. neil brown

    neil brown In Second Gear

    Messages:
    42
    Location:
    UK
    Anyone have any idea why my speedo reads fast, very fast, about 40mph at 30, and close to 80 at 55.. The truck has 16's, but if anything, this would make it read slow, although i now have some 14's with 195 70's for it, an they are they same overall height.
     
  2. pmrphil

    pmrphil In Overdrive GOLD MEMBER

    Messages:
    904
    The gear on the end of the speedo cable (in the trans) are available in different sizes/tooth count to make up for different rear gears, tire size, etc.
     
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  3. neil brown

    neil brown In Second Gear

    Messages:
    42
    Location:
    UK
    Thanks, sounds like an easy fix, just need to work out what gears I need now ..
     
  4. 72GTVA

    72GTVA Administrator Staff Member

    Messages:
    9,343
    Location:
    Chesapeake, VA
    The rule of thumb for speedometer driven gears is 3 MPH per tooth within the speed range that you noted (the 40/30 one), if you need to slow down the speedometer reading, you would need to get a speedometer driven gear that ADDs teeth, in your case 3 teeth to what you have in the transmission would get you closest.

    Reasons it could be off is a change in the rear gear or someone guessed on the replacement gear, or changed a transmission that uses a different speedometer driving gear (that is the one on the transmission output shaft, the speedometer driven gear is the one on the end of the speedometer cable).

    There is a relatively small range of gears and while you can usually get the speedometer close with just a change of the driven gear (the one on the end of the cable), there are times when the gear on the output shaft of the transmission has to be changed. This is typically caused by someone replacing a transmission with one from a compatible model but used a different power train engine/transmission/rear gear combination than what was in your model.

    Easy to figure out what gear you have - pop the single bolt holding the fork that retains the speedometer cable in the transmission, pull the cable out and count the teeth or look at the top of the gear for the numbers embossed on the gear.

    What transmission do you have, automatic or manual?
     
  5. neil brown

    neil brown In Second Gear

    Messages:
    42
    Location:
    UK
    Its an auto 'cruise-o-matic' 3 speed. A PO has changed the rear end to an Explorer 8.8, so this may be the issue. Thanks for the extra info on the gears, can I buy them from some where like Rock Auto? They are nice and easy for me to deal with in the UK.
     
  6. 72GTVA

    72GTVA Administrator Staff Member

    Messages:
    9,343
    Location:
    Chesapeake, VA
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  7. handy_andy_cv64

    handy_andy_cv64 In Maximum Overdrive SILVER MEMBER

    Messages:
    8,965
    Location:
    Everett, WA
    That Explorer rear end may have come from an AWD one, which usually has a 3.73 ratio. If an automatic overdrive had been put in, the speedo would likely read a lot closer. But you can shinny underneath, see if the axle tag is still attached to the rear cover. The bottom line of the tag will have the ratio, plus a single letter inside the ratio number, if it has a limited slip diff. For example, on my '97 AWD Explorer, the ratio reads "3L73." One without a letter code for the same ratio would be "3 73."
     
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  8. Doc76251

    Doc76251 In Fourth Gear

    Messages:
    347
    Location:
    Virginia Beach, VA
    I'm guessing you probably know this already but just in case...... Do your calculations in MPH not Km/H that way the "general rule of thumb" will work. Probably is due to what is stated above but the other thing that might have happened is that the PO converted to Km/H via the gear but missed it by a little bit. 30MPH = ~48.3 Km/H and 55MPH = ~88.5 Km/H. Food for thought and worth what you paid for it:cool:

    Cheers,

    Doc
     
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  9. neil brown

    neil brown In Second Gear

    Messages:
    42
    Location:
    UK
    Thanks all. the tag is still on the axle, i'll have a look.
    That does kinda sound right Doc, but we are mph over here as well, good idea tho. I'll crawl under the truck and do some maths.. Thanks again.
     
  10. uk ranchero

    uk ranchero In Second Gear

    Messages:
    49
    i got mine from PATC but you might get them from your local ford dealer as the same part is fitted in the A4LD
     
  11. dadcohn

    dadcohn In Second Gear

    Messages:
    29
    Location:
    Danville, CA
    Since you guys were talking about too fast speedometers...I just put in a new speedometer and cable on my 64, 3 on the tree. The needle on the new one won’t drop below 20 even when stopped. The old one never really worked at all. Any thoughts?
     
  12. colnago

    colnago In Overdrive

    Messages:
    752
    Location:
    Ridgecrest, CA
    With my '64, it was a bad speedo. I ended up getting a new unit from Scott Drake. I did try a couple from eBay (thought I could save a few bucks), but ended up with the Scott Drake unit. Also, in the process, I bunged up the cable, and had to get a new one. Make sure you lube the cable, otherwise, it will grab the housing, then let go, then grab, etc. This will cause the needle to bounce.

    Joseph
     
  13. andrewok1

    andrewok1 In Maximum Overdrive GOLD MEMBER SILVER MEMBER

    Messages:
    3,483
    Location:
    ocala ,florida
    sharp bend in cable will cause this problem
     
  14. burninbush

    burninbush In Maximum Overdrive

    Messages:
    8,286
    Location:
    near SF
    Bad speedometer unit.
     
  15. andrewok1

    andrewok1 In Maximum Overdrive GOLD MEMBER SILVER MEMBER

    Messages:
    3,483
    Location:
    ocala ,florida
    or as BB said ,
     
  16. handy_andy_cv64

    handy_andy_cv64 In Maximum Overdrive SILVER MEMBER

    Messages:
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    Location:
    Everett, WA
    Either one, but disconnecting the cable behind the head will tell you immediately if the head or cable's bad.
     
  17. dadcohn

    dadcohn In Second Gear

    Messages:
    29
    Location:
    Danville, CA
    Hmm, hopefully this isn’t a stupid question but how will I know which it is when I unscrew the cable? I went under and disconnected the cable from the transmission then rolled the gear until the speedometer needle was almost back to zero then re-installed it but it hopped back up to 20 again. Could it be the gear perhaps? Both the speedometer and cable are brand new right out of the box from Mac’s. I’ll get some cable lube and try that too.
     
  18. handy_andy_cv64

    handy_andy_cv64 In Maximum Overdrive SILVER MEMBER

    Messages:
    8,965
    Location:
    Everett, WA
    When you disconnect them, if the head is bad, the needle will stay at 20; a bad cable will allow the needle to drop to zero
     
  19. colnago

    colnago In Overdrive

    Messages:
    752
    Location:
    Ridgecrest, CA
    How so? The needle is driven by a rotating magnet. The faster it rotates, the higher the needle points. If it doesn't rotate, the needle should drop to zero. Disconnecting the cable shouldn't affect anything, since the magnet isn't rotating. Am I missing something (which is highly likely, since I haven't had my coffee yet)?

    Joseph
     
  20. handy_andy_cv64

    handy_andy_cv64 In Maximum Overdrive SILVER MEMBER

    Messages:
    8,965
    Location:
    Everett, WA
    It's likely the cable is pulling the input shaft sideways, but your point is correct--normally, the needle falls to zero when the cable stops
     

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