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Discussion Starter · #1 ·
Where to start?

I have a m21 4spd, 3.55:1 posi, 275 60R15 tires on
A 67 big block 4k car.

I calculated the required speedo
Gear to be 17 teeth - purple gear.

When I put it in and drove the car over 60,
My speedometer spun out of control. It spun to
120 mph and made a god aweful high pitched noise.

I disconnected it for now. This did not occur immediately.
I drove a few miles before it went crazy.

1) Any idea what broke or is faulty?

2) Anyone know who can repair this?
 

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Sosh - Could be a few things. Make sure the cable sheath is lubed, squeal often comes from the cable bit sometimes can be the actual odometer gears.

The speedometer may need some cleaning and lube as well as the cable.

Make sure the transmission side is also tight and seated right.

One test trick would be to disconnect the cable from the speedometer head, and drive where you think you are over 60, no squeal then it is in the speedometer unit. While disconnected also confirm you have cable spinning by just putting a dab of tape on the disconnected end and watch it while you drive.
 

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Well that is the big question, I'm thinking it was the internals of the speedometer head. You can test using a scrap speedo cable and a high-speed reversible electric drill, as most battery cordless drills don't have the rpms needed. Remove the speedo unit, secure it on a bench hook the section of the scrap cable to the head and other end securely in the drill chuck teeth let it rip. Of course make sure you have eye protection. If you can duplicate the sequel and peg your speedometer will need servicing.
 

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With a 4.10 I am using a 24 tooth speedo gear, so I would think you would need at least a 20 tooth. You can get a free app for your phone that will give your actual speed while driving down the road, it is a lot easier than having your buddy next to you and trying to keep at the same speed.

How tall are your rear tires?
 

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Discussion Starter · #6 ·
Tires are 26.5 inches tall. I am using GPS.

I have 3.73:1 gears and 275 60 R50 tires.

I have 17, 18,19,20,21, 22 and 23 tooth. The 21 tooth

Was way off!
 

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There are a couple great national shops, or try your local speed shop for advice. Bob's Speedometer is a fav of mine, also Red Line Gauge on the West coast does top notch work. Search the forums here and you should find a good thread from a while back on speedometer and tachometer rebuild shops.

Brian
 
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