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 Speedometer accuracy

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Mustangs Alley
Mach1 Enthusiast

Number of posts : 30
Your location : Garnet Valley, PA
Registration date : 2009-03-10

PostSubject: Speedometer accuracy   Wed 11 Mar - 12:46

Hi folks,
Question...not a gear head (just enough to get me in trouble and cost me more money than it should).....so please be kind!!

Since the rear diff was changed out by the previous owner, the speedometer is not accurate at all. It runs 10-15 mph higher than it should. Luckily GPS's are out that can help. However, I'd like to get it at least close, if possible. Any ideas on how this can be fixed? Can linkage be changed to accommodate? I'm hearing that you cannot get it accurate. I assume, if I put the 3.0 back in, it may be. But, I'm thinking of going with a 3.5 diff, which will most likely still cause the speedo to be incorrect.

Any suggestions?
PS: In states with strict inspection rules, an inaccurate speedometer will be a failing point !!! YIKES !!

Thanks again for your wealth of knowledge.

Kirk Question
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Mach1 Veteran

Number of posts : 158
Age : 57
Your location : Australia
Registration date : 2008-07-28

PostSubject: Re: Speedometer accuracy   Wed 11 Mar - 23:39

Hi Kirk,

you are active Very Happy I believe you can purchase different gears for the speedo cable to align with your rear end. I am not sure if you replace the gearbox end or the dash end so hopefully someone has done it before.

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Mach1 Member

Number of posts : 15
Your location : Australia
Registration date : 2010-04-21

PostSubject: New speedo gears   Mon 26 Apr - 6:09

Kirk, with regards to your speedo inacuracy. The driving gear attached to the end of the speedo cable is the one that is easily changed.
You can try all the various speedo calibration formulas on the net but I have never found them to work on Mustangs I have had.
The best rule of thumb is for every 5 mph you are out it equals one tooth on the gear. From memory if it reads too fast put more teeth in.
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