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Projects Speedo gear change

Discussion in 'The Hokey Ass Message Board' started by StarDiero75, Oct 14, 2017.

  1. StarDiero75
    Joined: Jan 15, 2017
    Posts: 170

    StarDiero75
    Member

    Howdy everyone!

    So I recently changed the rear end in my 65 Ranchero and I've noticed lately how no one drives the speed limit. I've been screaming at drivers because they're all driving 40 in a 50 or 22 in a 30 or 20 in a 25. But then after passing a speed limit sign that tells me what I'm doing, I've thought it's maybe my speedometer that's off.
    Does anyone here know how to do the calculations to how much less teeth I need to make the Speedo keep up with my speed?
    My tires are 25in, my trans is 3.03, it's a 200, rear end ratio coming soon.

    Thanks!
     
    chryslerfan55 likes this.
  2. e1956v
    Joined: Sep 29, 2009
    Posts: 2,376

    e1956v
    Alliance Vendor

    Run the Ranchero on a measured mile and see how many tenths your turning. Let's say your turning 11/10s in a mile, you speedometer would be reading 10% fast. Ford speedometer driven gears are 5% per tooth. So you would go 2 teeth more than the gear your running now.
    Hope this helps

    Sent from my SAMSUNG-SM-G890A using The H.A.M.B. mobile app
     
  3. StarDiero75
    Joined: Jan 15, 2017
    Posts: 170

    StarDiero75
    Member

    T
    That's a good idea.
     
  4. StarDiero75
    Joined: Jan 15, 2017
    Posts: 170

    StarDiero75
    Member

    So I switched from a 3.5 to a 2.8 guys. That explains why the car seems a little doggier.
     

  5. StarDiero75
    Joined: Jan 15, 2017
    Posts: 170

    StarDiero75
    Member

    Also it's a 20 tooth on the outside of the gear
     
  6. squirrel
    Joined: Sep 23, 2004
    Posts: 55,942

    squirrel
    Member

    so you might need a 16 tooth. But you ought to do the measure mile odometer test to be sure..
     
  7. StarDiero75
    Joined: Jan 15, 2017
    Posts: 170

    StarDiero75
    Member

    Actually I used a formula.

    Drive=drivenXrear ratioX(20186/tire diameter)/1001. I got 16 exactly. Math don't lie man. Haha. I used it on the original ratio first to make sure it was right.
     
  8. squirrel
    Joined: Sep 23, 2004
    Posts: 55,942

    squirrel
    Member

    If it was reading correctly before the change, then that should work. But if it was off a little, it will still be off a little with the 16 tooth gear. I'd check it first, just to make sure.
     
  9. StarDiero75
    Joined: Jan 15, 2017
    Posts: 170

    StarDiero75
    Member

    It used to be pretty dead on before so I'm not too worried.
    Thanks for the input man!
     
  10. coilover
    Joined: Apr 19, 2007
    Posts: 696

    coilover
    Member
    from Texas

    To hell with all the number crunching, just use a gps to get a DEAD ACCURATE speed and compare this as a percentage to the speedo reading. Use this percent number to choose the drive/driven speedo gear teeth that will either raise or lower the reading by that percent---done. I have had so may combos in my old 37 I finally just bought a gps speedometer and it doesn't give a shit about tire sizes, gear ratios, or anything else.
     
    firstinsteele likes this.
  11. squirrel
    Joined: Sep 23, 2004
    Posts: 55,942

    squirrel
    Member

    That sure takes the fun out of it.
     
    firstinsteele likes this.
  12. StarDiero75
    Joined: Jan 15, 2017
    Posts: 170

    StarDiero75
    Member

    Yeah man. That takes the fun out of it, haha. There ain't nothing like cracking out the old pad and paper and figuring crap like this out.
     
    earleg likes this.
  13. ol-nobull
    Joined: Oct 16, 2013
    Posts: 1,655

    ol-nobull
    Member

    Hi. I have a 46 Chevy coupe that is 10 MPH slow on spedo reading at 55 MPH I am going 65 MPH GPS. I changed the 4:11 out and installed a 3:55 ring & ponion and changed rear tires from 600 16 to 700r 16 and this is what is causing the 10 mph difference. The Coker catalog shows the 600 16 as 28.3" diameter and the 700r 16 at 30.18" diameter. I know they make the mechanical ratio adapters that screw on the transmission and spedo cable screws onto the adapter. Does anyone know the proper adapter # and anyone who sells them.
    I know Patrick's has them but I am not about to go there again from past problems with them.
    Thanks, Jimmie
     
  14. e1956v
    Joined: Sep 29, 2009
    Posts: 2,376

    e1956v
    Alliance Vendor

  15. Engine man
    Joined: Jan 30, 2011
    Posts: 3,480

    Engine man
    Member
    from Wisconsin

    All of the time and money I've spent fixing mechanical and inductive speedometers and I see no need for them with the availability of GPS speedometers for $30 that can even project on your windshield.

    I see there is a product called Classic Speed with a motor to connect via cable to the original speedometer that is operated using a GPS signal. Seems they have had more orders than they can fill and are back ordered. They cost around $300.

    https://www.classicautoinnovations.com/product-page/classic-speed-early-adopter-unit
     
  16. squirrel
    Joined: Sep 23, 2004
    Posts: 55,942

    squirrel
    Member

    the digihud app is free, I used it in the Corvair when the speedo died a grisly death not far into my 5000 mile trip. It sucks.

    I much prefer using a real speedometer, and one that has a little bit of character, ie a bouncing needle, reading a bit slow or fast at some speeds, and hard to read odometer numbers, just make it more betterer.
     
  17. e1956v
    Joined: Sep 29, 2009
    Posts: 2,376

    e1956v
    Alliance Vendor

    We carry Classic Speed.
    They just finished the gen II
    model and are in production now. Backordered units will soon be filled. It is a great product. I installed one on my 56 Vette and did a thread on the unit and the install. You can search Classic Speed on the board and find the thread. It's great for T5 conversions that do not have a mechanical speedometer drive.

    Sent from my SAMSUNG-SM-G890A using The H.A.M.B. mobile app
     
  18. StarDiero75
    Joined: Jan 15, 2017
    Posts: 170

    StarDiero75
    Member

    I feel exactly the same man. Nothing like a crazy needle at 60. It adds more to the thrill and laughs of an old car
     
    squirrel likes this.
  19. ol-nobull
    Joined: Oct 16, 2013
    Posts: 1,655

    ol-nobull
    Member

    Dang right. I like betterer also. Jimmie
     

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