How to diagnose speedometer head issue

[tadkingnkc]

So I have a stock 84 speedometer head and my speedo has always bounced and been inaccurate for my tires. So, I finally buy a Brand new speedo cable from ATP and driven gear for the 700r4 to fix both issues. I have triple checked drive gear 18tooth , driven gear 39 teeth , tire diameter of 34.9 with TCI Auto so my parts are all correct. Speedo reads about 7-8 mph OVER actual and brand new cable is noisy at the head inside the cab. Also above 60mph, it just runs up to the max gauge well above 85mph but that is not the case. So frustrating. Do I need a new speedometer?

 

[SirRobyn0]

So I have a stock 84 speedometer head and my speedo has always bounced and been inaccurate for my tires. So, I finally buy a Brand new speedo cable from ATP and driven gear for the 700r4 to fix both issues. I have triple checked drive gear 18tooth , driven gear 39 teeth , tire diameter of 34.9 with TCI Auto so my parts are all correct. Speedo reads about 7-8 mph OVER actual and brand new cable is noisy at the head inside the cab. Also above 60mph, it just runs up to the max gauge well above 85mph but that is not the case. So frustrating. Do I need a new speedometer?

The noise is because the speedometer cable is to long. Not the sheath but the cable inside. If it's to long it'll put pressure on the speedometer head causing bounce, or noise or both. Take the cable out of the sheath and grind an 1/8" off the speedometer end, reassemble and retest. Be careful not to take to much off or you'll ruin the cable.

Speed discrepancy. Are you taking into account the rear end gearing? And if so are you SURE you know the ratio.

Yes a bad speedohead can cause issues, but be sure of everything else first.

 

[tadkingnkc]

Yes, I just installed a 3.73 ratio a couple of weeks ago. Paired it with a 39 tooth driven gear, verified I had a blue 18 drive gear, and a new cable. Tire diameter is 34.9. TCI says everything is correct. Speedo acting wonky. What’s the best way to trim the cable end? can I use tin snips?

 

[SirRobyn0]

Yes, I just installed a 3.73 ratio a couple of weeks ago. Paired it with a 39 tooth driven gear, verified I had a blue 18 drive gear, and a new cable. Tire diameter is 34.9. TCI says everything is correct. Speedo acting wonky. What’s the best way to trim the cable end? can I use tin snips?

Bench grinder. Don't snip it you'll leave a jagged edge. If you don't have a bench grinder you could carefully put the cable in a vise and use a hand held grinder or a file, but a file will take a while. Maybe shortening the cable will solve all your issues, but I'm betting the inaccuracy is another issue.

I know you have a new cable and this thread https://www.gmsquarebody.com/threads/speedometer-cable-service-how-to.37207/ Is mostly focused on a used cable but there is a bit in there about shortening cables.