Speedometer needle shakes (and is off by @15 mph)

[JOMIDABR]

Anyone know a fix for the speedometer needle shaking and also off by @ 15 mph. 440 motor with 727 torqueflite trans. Also, looks like speedo cable at trans is leaking a bit.

 

[texas69bee]

http://brewersperformance.com/products.asp?cat=276

 

[HEMI-ITIS]

It might be the speedo gear but it's more likely the speedo cable is bad.

 

[Scooby67]

Off up or down? At 60MPH, 1 tooth = 2 – 3mph. More teeth slow the speedometer down, less teeth raise the the speedometer. Take your tire diameter and multiply is by your rear end ratio. This will give you the number of teeth your speedo gear should be. When you pull your gear, it will be stamped on it. Also, the speedo gear housing on the 727 is clocked, depending on what size gear your installing. There are 3 positions, if I remember correctly.
I got my speedo gear here: http://laspeedometergear.com Brewers charges double!!

 

[Meep-Meep]

Certainly look at the speedo cable but it can also be the jewel movement in the speedo head. The aluminum cup with spinning magnet is pretty precise and after many years things wear out - and rightfully so. You might even get away with some oil on the bronze bushing in the back of the speedo head.

 

[JOMIDABR]

Speed is off high by 10-15. If I'm doing @ 45, Speedo shows @ 60. Thanks for the tips all. Time to go to work and see what I can find.

 

[tallhair]

Not speaking to the shaking speedo needle but it's more than likely the gear is wrong with that much difference in speed vs actual Mike. I saw in another thread the trans is not original to the car so it probably has the gear from whatever car the trans came from.

What rear gear do you have 3.23?

 

[JOMIDABR]

Thanks Tall Hair. Not sure on the rear end. I purchased this car from an estate sale, with limited information. It's an interesting car that I feel I purchased at a decent price ($40,500), regardless of numbers matching. I do know that it is a true RT convertible based on the vin starting with WS27....Trying to get my arms around what I have here. Do you know how I would confirm the ratio of the rear end?

 

[tallhair]

Jack it up and support the rear with good jack stands for safety and see if the ratio tag is still on one of the bolts that hold the rear chuck in the axle housing. If it's not there make sure the car is in nuetral and have someone rotate the tires one revolution exactly and count the revolutions of the drive-shaft. If the rear has never been out it may have a sure-grip tag. If both wheels turn the same direction its a sure-grip. Should also be on your fender tag if it's a sure-grip car.

2 3/4 = 2.76
2 and almost 3 = 2.94 (think that's the right 8.75 ratio)
3 1/4 = 3.23 which was standard ratio
3 1/2 = 3.55
3 and almost one = 3.91
4 and a little bit = 4.10

Also could drive it 60 mph (get a pace car to work with you) and see what the RPM are if you have a tach and we can figure it out based on your tire size

 

[JOMIDABR]

Found the gear ratio this morning. There was a small metal tag on one of the bolts in the rear end. It is 3.23, which as you said, I believe to be original. I'm going to try a few of the suggestions that you all posted on this thread.

 

[A383Wing]

you don't say what year you are working on or what kind of car...for the needle bounce, try lubing the cable, if that does not help, you will need a new speedo head.

 

[1966satellite]

There is a speedometer app for your phone that you can use to determine your actual speed. The one I use is Speed Box.

 

[JOMIDABR]

Sorry...it's a 68 Coronet R/T Convertible, 440 Magnum. Do you need to take the cable out to lube it. Also, what type of lubricant do you suggest.

 

[A383Wing]

disconnect cable casing from back of speedometer, pull down from under dash, pull cable out of casing......use lightweight grease to lube cable as you re-install it into the casing. Keep adding grease to cable as you turn and install back in case