Close



Results 1 to 4 of 4
  1. #1

    Default 1985 Ford Mustang GT - Speedo Off - New Driven Gear

    Fellow Fox Body Fans,

    I have a 5-speed manual transmission in my 1985 Ford Mustang GT. My speedometer was off by about 8-9 mph. When the speedo is showing 57, my GPS is showing me doing 65 mph. It gets a little more off as I go faster (as expected).

    I pulled out the current speedo driven gear and it is 16T. The LMR.com calculator and CJPony Parts calculators stated I should be using a 17T. So, I bought a 17T and installed it. The speedo is off by a much larger margin now. At 55 mph on the speedo, the GPS is showing me at 65 mph.

    I looked at the American Muscle chart and it shows I should be using an 18T. Will an 18T make it worse?

    As far as I know, this is the stock T5 transmission for the car with the 3.08:1 rear end gear. My tires are the same size as stock and have about 870 revolutions per mile.

    What is my next step? Speedometer cable? Speedometer gauge? Thanks.

    JB

    1985 Ford Mustang GT, 8.5K original miles
    USAF, CMSgt (ret), 1985 - 2015
    Last edited by Huskers001; 08-09-2019 at 08:22 PM.

  2. #2

    Default

    57mph/65mph = 0.877. That is the error in your speedo.

    Since your speed is going too slow, so you need smaller driven gear, not a larger one.

    With the above speed, you were using a 16t driven gear so:

    16t * 0.877 = 14.03t

    That is the gear that you need. However, it doesn't exist. I suspect that something is wrong with your speedo. No gear can fix that.
    Jack Hidley
    Maximum Motorsports Tech Support

  3. #3

    Default

    Quote Originally Posted by Jack Hidley View Post
    57mph/65mph = 0.877. That is the error in your speedo.

    Since your speed is going too slow, so you need smaller driven gear, not a larger one.

    With the above speed, you were using a 16t driven gear so:

    16t * 0.877 = 14.03t

    That is the gear that you need. However, it doesn't exist. I suspect that something is wrong with your speedo. No gear can fix that.
    Jack,

    Thank you for the explanation. I understand where I went wrong with going with additional teeth on the speedo driven gear.

    Unless you have another recommendation, I plan to start with the speedo cable although I suspect it's not the culprit. For now, I plan to put the 16T back in because it's not as far off as the 17T. If it is the speedo cable/gauge itself, I should see the difference go the other direction when I correct the problem...correct? That would mean I'd go back to the 17T which is what the specs state for this setup.

    Thanks again.

    JB

  4. #4

    Default

    Its impossible that the problem is the speedo cable.

    The cable either has a 1:1 speed ratio (input to output) or it is twisting up like a spring while the car is moving. If it were doing this, then it would fail after 20 seconds or so.

    You could also install a larger gear on the transmission output shaft, but this requires removing the tailhousing.

    Yes to your question.
    Jack Hidley
    Maximum Motorsports Tech Support

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •