speedo reads fast

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

  1. neil brown

    neil brown In Second Gear

    Messages:
    39
    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:
    872
    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:
    39
    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:
    9,293
    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:
    39
    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:
    9,293
    Location:
    Chesapeake, VA
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  7. handy_andy_cv64

    handy_andy_cv64 In Maximum Overdrive SILVER MEMBER

    Messages:
    8,717
    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:
    340
    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:
    39
    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:
    43
    i got mine from PATC but you might get them from your local ford dealer as the same part is fitted in the A4LD
     

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