Speedometer question

[jrm27]

On my 67, everything (as far as I know) is stock. But apparently something was changed before I bought it. A few miles from where I live is an odometer check area. When I try to match up my odomoter to the mile markers I find I have a problem. When the marker says one mile, my odometer says that I've gone only 9/10 of a mile. This 10% difference is noticable in my speedometr as well... my needle will say 50, but I'll get clocked at about 55 or so by the roadside speed checker or my GPS. Any ideas on what I should look at? basic question I know, but any help would be appreciated.

-jrm

 

[J-Scratch]

Atleast you know mentally. Keep the miles down.
Sorry - no real help.

 

[65and68Stang]

First thing that comes to mind is the speedo gear. Transmissions, tires sizes, and rear end ratios will all lead to changes in the speedometer readings. There is a little plastic ring with teeth that goes on the end of the speedo cable into the transmission. Change the number of teeth, change the speedo reading. Unfortunately, I don't know whether to tell you to go up or down on the number of teeth, but someone here does I'm sure.

C'ya!

Kelton

 

[Max Power]

The number of teeth on your driven gear depends on your drive speedo gear. The drive gear is probably six or seven. The formula is # of teeth on drive gear(6 or 7) x axle ratio x tire revolutions per mile divided by 1000.

That number will be closest to the number of teeth needed on your driven gear.

Order a National Parts depot catalog. They have the explanation there as well as a variety of gears. Some swirl the teeth to the left, some to the right, so you need their catalog to know what you are doing.

I had no clue how many tire revolutions I had to the mile, but I knew my tire height was 26 unches, so I called Tire Rack and they told me the number. NPD might be able to as well.

Hope that helps.

 

[68RCodeConv]

Your speedo/odometer is running "slow"....

Meaning that it is registering LESS than actual. If it is showing exactly .9 for one mile then it is running about 11% slow. Pull the speedo cable out of the tranny and count the teeth on the gear. You need a gear with FEWER teeth to make your speedo/odo run faster. If you currently have a 20-tooth gear, an 18-tooth gear will make your speedo very close (20/18 = 1.11 = 11% faster). If you have a four-speed, you will need a gear for a four-speed. Autos and 3-speeds use the same gear.