Speedometer Issue

[warrenbh]

I have a speedometer that grossly understates my speed. If i'm on the highway moving with the flow of traffic (no jams), the speedometer tells me i'm going 35 when it should say something closer to 65-70.

Do i need a new speedometer or is this a simple fix on a 66 mustang gt.

Thanks in advance for any responses.

 

[tx65coupe]

All you should need is a different speedometer gear. It goes on the end of the speedo cable at the transmission and can be changed out in a few minutes. I bet the car does not have the original rear end gears and/or has larger rear tires.

Before you buy a speedo gear, you need to know what the rear gears are and what size the rear tires are.

 

[D.Hearne]

Did it just start doing this on it's own? Without your changing the rear gears ? Or tires ?

 

[2+2GT]

Um, yeah, before you rip it apart, is the odometer accurate? Use the mile markers at the edge of the road. Chances are though, you will need a different driven gear on the end of the cable.

 

[calpolymustang]

if the rear end is a lot different that stock type ratios, you may need a gear reduction device that plugs into the tramission.

when i added the 4.11s, i had to get a gear reduction since speedo gears with more than 21 teeth dont last long before wearing out.

 

[68RCodeConv]

if the rear end is a lot different that stock type ratios, you may need a gear reduction device that plugs into the tramission.

when i added the 4.11s, i had to get a gear reduction since speedo gears with more than 21 teeth dont last long before wearing out.

He does not need the gear reduction. That is for when the speedo reads HIGHER than actual speed. He needs fewer teeth on his speedo gear to "speed up" the speedo.

That being said, I have never heard of a speedo reading that low (almost 50% low). If you currently have a 20 tooth gear then it means you would have to go to something like a 12 tooth gear which does not exist.

I suspect something is wrong with the speedo.