speedo too slow

[1985 5.0]

ive got a new tranny (2 months) out of a turbo tbird.....i had my dad to get in front of me to make sure our speeds were matching well they werent when he was goin 60, mine was showin me goin 50 this could have been a major problem as i have passed many cops goin 70 when i thought i was goin 60 :damnit: what all would i need to do as far as gettin a speedo cable to get it runnin correctly?

 

[seijirou]

You need to determine if you are a constant 10 mph off, or if it's gradiant. such as 5 mph slow at 30, but 10mph slow at 60.

If it's a constant 10 mph problem, your needle on your guage needs to be adjusted

if it's gradient, then you need to change the gear on the speedo.

 

[1985 5.0]

its basically 10 mph consistenly when he was goin 35 mine was showin 25 but i didnt check it no lower than that i will have to do this again tommorow with my moms stang

 

[seijirou]

You don't happen to have white face guages or something do you.

This probably isn't gearing related.

On the newer model mustangs the needles just pull off. On your 85 however i know it's a little bit different. I have a guage cluster like yours sitting here in my basement, I'll take it all apart and see what's going on in there. I don't know if the needles can be adjusted or not.

Maybe someone else who knows the 4 eye car's guages will chime in sooner than it'll take me to see what's what though

 

[bobcatv8]

Speedos are not "off" by mile per hour, they are off by percentage. Other wise your speedo would be at a constant MPH error at any speed. What it sounds like is you have a different number of teeth on the drive gear on the new trans you swapped in. You can adjust this somewhat by changing the driven gear {the gear on the end of the cable}. Gears come in tooth range 16 to 21. See what's in there now and try to adjust it out that way first, gears are cheap, $5-$8 each. If you need to "speed up" your speedometer, try a gear with 2 less teeth if possible. Obviously, if you have a 16 or 17 tooth gear already on, you can't adjust it out that way. You may have to take it to a speedometer calibration shop where they can install an inline reduction box that will calibrate it properly. That won't be cheap.

 

[1985 5.0]

i dont have white gauges ... then i hope that this cable has more than 16-17 teeth b/c im not payin out the ass to take it to a shop to recalibrate it

 

[bobcatv8]

Even one tooth down would get you closer. Also see if you can swap drive gears on your tranny. Get one with the same # teeth as your old tranny and your'e back in business.
On AOD's the drive gear is made on the output shaft, not replaceable. No one ever checks this before they swap AODs, they are either 7 or 8 teeth and if you don't put the same number back, the speedo is off. I see this all the time at work.

 

[I_AM_CANADIAN]

Speedos are not "off" by mile per hour, they are off by percentage.

In my 86' ranger, I replaced the dash cluster and the speedo was a consistent 10km/h off. I adjusted the needle as such and it was accurate again.

 

[baldy1]

What color is your gear on the end of your speedocable. If you havent done anything but change the tranny. Find out the tooth count on your original tranny. You cant changed that cause its part of the output shaft of the tranny. Which is either 7 - 8 teeth

teeth color gear

16 blue
17 green
18 gray
19 tan
20 orange
21 purple

 

[Fishhawk]

As started earlier. Its your speedo driven gear. Pull the cable out of the trans. All the speedo cable driven gears are color coded. Whatever tooth gear you have, move up or down to the next one. Very simple and inexpensive. Baldy's got it for you. Next stop your Ford dealer.