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  1. #1

    Default 88 v8 Bird speedo is way off

    I have an 88 Sport Coupe Thunderbird, ever since is got the car the speedometer has been off. 35 is 55, 55 is 75. and so on and so forth, the speedo is the stock 95mph, stock trans, and a 3.73 rear gear. The gear in the tail shaft is a 19 fir the sensor and a 7 if I counted it right. This is very annoying when I let someone drive the car, and they complain about the speedo, and I have to say that it is very aggravating not knowing my speed. I could really use help with this.

  2. #2
    Moderator wraithracing's Avatar
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    Default

    More information is needed to help verify what speedometer gear is needed.

    Check this out: https://lmr.com/products/Mustang-Spe...ear-Calculator
    ​Trey

    "I Don't build it hoping for your approval! I built it because it meets mine!"

    "I've spent most of my money on Mustangs, racing, and women... the rest I just wasted."

    Mustangs Past: Too many to remember!
    Current Mustangs:
    1969 Mach 1
    1979 Pace Car now 5.0/5 speed
    1982 GT Ongoing RestoModification
    1984 SVO Awaiting Restoration
    1986 GT Wrecked by PO, but still want to save!

    Current Capris:
    1981 Capri Roller
    1981 Capri Black Magic Roller Basket Case
    1982 Capri RS 5.0/4spd T-top Full Restoration Underway
    1984 Capri RS T-top Roller
    1983-84 Gloy Racing Trans Am/IMSA Body Parts

  3. #3

    Default

    Depending on the tire size, the 7 tooth with a 19 or 21 would already be as correct as it could. Also, the stock trans should have been an 8 tooth, as the stock rear was a 2.73 or 3.08.

    If you really wanted to, I could pull the cluster out of my 88 xr7 and you could swap it out to see if its the cluster or not.
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

  4. #4
    Moderator wraithracing's Avatar
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    Default

    The issue here for me at least is the lack of complete information.

    I am not the TBird expert, so help me out here. In one thread you are asking about a 2.3T to 3.8 swap. So is this TBird the same or different as the title here states a V8 car. Also in the other thread the transmission is an automatic. So what is this car again manual or automatic.

    The next needed information would be what size tires are currently on the vehicle?

    Last is how do you know the rear gears are 3.73? Did you install them, did the previous owner, are these stock gears, did you verify the gear ratio and if so how?
    ​Trey

    "I Don't build it hoping for your approval! I built it because it meets mine!"

    "I've spent most of my money on Mustangs, racing, and women... the rest I just wasted."

    Mustangs Past: Too many to remember!
    Current Mustangs:
    1969 Mach 1
    1979 Pace Car now 5.0/5 speed
    1982 GT Ongoing RestoModification
    1984 SVO Awaiting Restoration
    1986 GT Wrecked by PO, but still want to save!

    Current Capris:
    1981 Capri Roller
    1981 Capri Black Magic Roller Basket Case
    1982 Capri RS 5.0/4spd T-top Full Restoration Underway
    1984 Capri RS T-top Roller
    1983-84 Gloy Racing Trans Am/IMSA Body Parts

  5. #5

    Default

    I have several TBirds, and this is a different TBird. This is a V8 Sport Coupe, the trans is automatic, AOD. 3.73 postreaction rear end (from a 1987 turbo coupe auto. read the tag on the diff) , tire is 285 35 R18. I hope this helps.

  6. #6

    Default

    Well the math says a 20 tooth would be correct. 1 tooth isn't going to make up 20 mph at 55 though.
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

  7. #7

    Default

    I can change the gear pretty easily, but what are the chances that the sensor is bad? Reading wrong? I just had the trans rebuilt and the car is still in the air, so it will be easy to get to.

  8. #8

    Default

    Correction, the sensor gear is an 18. I pulled it and recounted.

  9. #9

    Default

    It would correct it by about 10mph at 60 if you through in a 20 tooth.
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

  10. #10

    Default

    The gear is on the way, I will leave an update as soon ass it comes in.

  11. #11

    Default

    Gear came in, took her down the road for the first time in about a year, trans shifted good, but the speedo is of by 5mph, whole lot better then what it was.

  12. #12

    Default

    5 mph at what speed. Which direction — reads higher or lower than vehicle speed?

    you add teeth to slow sensor/cable rpm (and make the speedometer read slower)

    you reduce teeth to speed up the rpm

  13. #13

    Default

    I've got the same issue in my Fairmont...where 54mph on the speedo = 65mph in real life. Had no clue when I was driving it home after I bought it. But I plan on addressing after I do the rear end swap and new wheel/tire combo. I didn't see anything in the links above in regards to C4 cars. Any help would be appreciated.

  14. #14

    Default

    It's 5 high, and that is throughout the speedo, no change at any speed, it stays a constant 5 higher then the actual speed of the car. 45 on the speedo is 40 actual mph and this is with the 20 tooth gear

  15. #15

    Default

    Sounds like something in the speedo has slipped.

    when tire size or speedometer gearing cause things to be off they get more off the faster you go.

    For example, with a 235/60/15 the car will be moving 92 when it says 85 but only 65 at 60, etc.

    Check your odometer vs the white marks on the highway bear in the air use to time us. Each tick is 1/2 mile. Or use mile markers.... if your odometer is on but speedometer is off that is a different type of problem then both being off

  16. #16

    Default

    Exactly. If its off at 5mph doing 10mph, then it should be off by 20mph at 40 mph.
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

  17. #17
    Moderator wraithracing's Avatar
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    Default

    First and foremost, the speedometers on these cars are not exactly the most precision built unit on the planet. There is play and slop in there that is just the norm and can't really be altered without major work. With that said, it's not uncommon due to the sloppy nature of the speedo to be off a bit at different speeds. In my experience the best option is to test the speedometer at 60 miles per hour if possible. This is a good speed because you can check both the odometer and the speedometer at the same time to see how accurate your misc gears are. Best if you can use mile marker signs on the highway to help with your calculations and measurements too.

    I would test the current setup at 60 MPH on the highway and adjust from there if needed. If the speedo is still WAY OFF at other speeds or all other speeds, than I would seriously consider swapping it for a known good speedometer and retest. Good Luck!
    ​Trey

    "I Don't build it hoping for your approval! I built it because it meets mine!"

    "I've spent most of my money on Mustangs, racing, and women... the rest I just wasted."

    Mustangs Past: Too many to remember!
    Current Mustangs:
    1969 Mach 1
    1979 Pace Car now 5.0/5 speed
    1982 GT Ongoing RestoModification
    1984 SVO Awaiting Restoration
    1986 GT Wrecked by PO, but still want to save!

    Current Capris:
    1981 Capri Roller
    1981 Capri Black Magic Roller Basket Case
    1982 Capri RS 5.0/4spd T-top Full Restoration Underway
    1984 Capri RS T-top Roller
    1983-84 Gloy Racing Trans Am/IMSA Body Parts

  18. #18

    Default

    Tbirds use a fully digital Speedo input. I can get mine within 1 mile an hour accurate by GPS up to 120mph. The tire sizes are usually off more then the Speedo. Even the analog gauge is driven digitally.
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

  19. #19
    Moderator wraithracing's Avatar
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    Default

    Quote Originally Posted by Haystack View Post
    Tbirds use a fully digital Speedo input. I can get mine within 1 mile an hour accurate by GPS up to 120mph. The tire sizes are usually off more then the Speedo. Even the analog gauge is driven digitally.
    I didn't even think about that when posting above. I was think about the standard Mustang/Capri speedo.

    So the T Bird doesn't use any speedometer cable at all? If that is the case, then either the driven gear tooth count is still off or there are electronic issues.
    ​Trey

    "I Don't build it hoping for your approval! I built it because it meets mine!"

    "I've spent most of my money on Mustangs, racing, and women... the rest I just wasted."

    Mustangs Past: Too many to remember!
    Current Mustangs:
    1969 Mach 1
    1979 Pace Car now 5.0/5 speed
    1982 GT Ongoing RestoModification
    1984 SVO Awaiting Restoration
    1986 GT Wrecked by PO, but still want to save!

    Current Capris:
    1981 Capri Roller
    1981 Capri Black Magic Roller Basket Case
    1982 Capri RS 5.0/4spd T-top Full Restoration Underway
    1984 Capri RS T-top Roller
    1983-84 Gloy Racing Trans Am/IMSA Body Parts

  20. #20

    Default

    Yup, no Speedo cable at all.
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

  21. #21
    Moderator wraithracing's Avatar
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    Default

    Quote Originally Posted by Haystack View Post
    Yup, no Speedo cable at all.
    See one more reason why I generally don't comment on T-Birds since I have never owned one or worked on one!

    Maybe some day I will get my 87/88 Turbo Coupe . . . . . Probably NOT!
    ​Trey

    "I Don't build it hoping for your approval! I built it because it meets mine!"

    "I've spent most of my money on Mustangs, racing, and women... the rest I just wasted."

    Mustangs Past: Too many to remember!
    Current Mustangs:
    1969 Mach 1
    1979 Pace Car now 5.0/5 speed
    1982 GT Ongoing RestoModification
    1984 SVO Awaiting Restoration
    1986 GT Wrecked by PO, but still want to save!

    Current Capris:
    1981 Capri Roller
    1981 Capri Black Magic Roller Basket Case
    1982 Capri RS 5.0/4spd T-top Full Restoration Underway
    1984 Capri RS T-top Roller
    1983-84 Gloy Racing Trans Am/IMSA Body Parts

  22. #22

    Default

    You should. They are pretty good cars. Usually a lot cheaper then mustangs, I. Better shape (for the price) have way more room and are much more comfortable.
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

  23. #23

    Default

    Send it!

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