Odometer right/speedo wrong

[RocketMan]

How can this be? '75 H/O, the odometer is dead on perfect, but the speedo is off depending on how fast the car is moving.

If the speedo says 55, the car is going closer to 60 -62. If the speedo says 60, the car is going about 70. If the speedo says 70, the car is going about 83-85 :confused: . Any ideas?

[ragz442]

Did you change rear gear ratio or rear tire size? May be the cable, does it make noise at all?

[sledhead2]

did you swap out the trans? or have any trans work done?

[RocketMan]

Nope, didnt do anything with the rear or trans. The cable dosent make any noise either :confused:

[sledhead2]

How long have you owned the car? Was it off from day one you have owned it? Do you know what the rear gear is?

[ragz442]

Does it have speed [cruise] control?

[442MUCH]

What are you verifying the speed with? Are you sure that the instument of measure (friend's car) is correct? What is your transmission and engine?

[keith455]

Get a gps and see how much it's off. How do you know the odometer is correct?

[RocketMan]

Yeah I've had the car awhile. Rear is 2:73, no cruise, 455/400. The odometer is scary correct, I have checked it every way I can - mile markers, distance signs, fuel mileage, driving direction - its perfect.

The speedo is off, the State Trooper confirmed it clocking me @77 when I thought I wwas around 65. Later confirmed with a new Solara. Its more than a mile or two off.

[sledhead2]

you need to post your tire size that way we can put info into a web program for figuring out trans gears. My guess is some were along the cars life some one put in a different trans and the speedo gear are wrong in the trans.

[ragz442]

you need to post your tire size that way we can put info into a web program for figuring out trans gears. My guess is some were along the cars life some one put in a different trans and the speedo gear are wrong in the trans.

Sounds possible, but he says nothing was done to the trans. Has it always been off?

It's a very simple speedo system. Like Dave said, depending on tire size and gear ratio, that will determine gear size [color].

I've got 4.10's and 27560's, I went to my local trans parts house, he plugged the info into his puter and I walked out with what I needed for less than $10. I did replaced the cable too, It would bounce around alot and because it was so sloppy, it threw the reading off.

[442MUCH]

OK. You have a 455 and 2.73 gears. If you have the "OD" Turbo 400, you should have a RED #37 Driven gear running GR70-15 (whatever that is today) tires. The OD transmission is for all states except California.

See if you have the RED driven gear. And whatever GR70-15 works out to, will let you know what tire size you should be running. I believe that 235-70-15 is the correct tire size.

You could also apply white grease to the cable and use a drill to turn it. Just a few ideas.
Good luck.

http://img.photobucket.com/albums/v417/vttedrm/drivengear.jpg

[RocketMan]

Well the other day I was talking with a professional auto restorer, and this topic came up. He instantly had the answer - or so I hope :)

Without batting an eye he said it was probably the "hair spring" or "clock spring" in the speedo. He had a disassembled speedo laying around and showed me how they work. The odometer is geared directly to the trans via the speedo cable. The speedo works by way of a spinning magnet that moves the needle by twisting a small spring ( similar to what you would find in a watch or clock ). If the spring is tight the speed displayed is too low, if its loose the speed shown is too high.

Now I have to find a place to calibrate the spring. :eek:

[76CutlassS]

Here ya go,

This one will tell the difference in your speedo reading from one tire size to another. Try it.

http://www.miata.net/garage/tirecalc.html

Tire Size Conversion Chart

http://www.tirerack.com/tires/tiretech/45_conversionchart.html

Speedometer gears

http://www.tciauto.com/Products/TechInfo/speedo_gears.asp#400

The chart tells you what the drive gear should be based on the color. When you remove the piece that holds the driven gear shine a flash light inside the transmission and see if you can tell the color of the drive gear then look at the chart and it should tell you the number of teeth based on the color. Then go to the calculator at the top of this web page and enter the number of drive teeth, your rear-end gear ratio and tire size diameter hit calculate and it should tell you the driven gear you need


General Motors Speedometer Gears

http://www.tciauto.com/Products/Components/speedometer_gears.asp#gm

-harv

[58thndy]

Find a good speedometer repair shop (I'm bound to think there's at least ONE in NYC) and have them bench calibrate it. It'll require removal, but that's a ten minute job on these cars.

You've seen that the odometer is pretty much a direct drive off the cable, so that's why your odometer reads right. The error is in the magnetic field driving the speedometer. It's an induction motor, for all practical purposes- the magnets spinning in their cage induce a magnetic field in the hairspring which causes it to move with the spinning magnets.

What they will do is zap the magnets to where they are either stronger (to drive a tight spring) or weaker (for a loose spring).

[442MUCH]

BTW, the correct conversion from GR70x15 is 225 70x15 :)