speedometer calibration

[firsttimer]

Hoping for some help.
Have been unable to get my speedometer to calibrate>
I have a tremec TKO600 with a 347 stroker (carburated) and using the RF wiring harness and Factory Five vintage guages.
The speedo sensor is plugged into the port on the transmission (mid passenger side) and has the grey and green 'speedo sensor' wires.
the dash harness has these same two wires, however the speedometer itself seems to have a yellow wire with black stripe ( I assumed ground) and a yellow wire with red stripe. The wiring diagram shows the green wire from the harness going to the speedo so I attached it to the yellow/red wire of the speedo. Unsure where the grey wire from the trasmission/wiring harness goes (one of the wire diagrams refers to EFI sensor but running carburator)
I know the speedometer is getting a signal because the needle moves to 300km/h when plugged in. Going thru the calibration process changes nothing with the needle wandering all over the guage.
All the other guages work great.

I know I am missing something, hopefully something simple.
once this sorted out off to paint!!!

thanks
Any help greatly appreciated

Mike (firsttimer)

[Doutie]

Hi, I'm Doutie and I'm a mechanic by trade. My roadster has the vintage gauges but I didn't build it and can't help you with the wiring by color but I can help you understand the speedo and how it works. The speedo counts electrical pulses generated by the sensor at the transmission. Inside the sensor is a coil of wire and the two wires, or connections coming out of it are the ends of the wire coil. The tip of the sensor is placed close to a special gear on the output shaft. As a tooth of this gear approaches the sensor it cause a positive voltage in the coil. As the tooth passes the sensor and moves away it makes a negative voltage. These voltage pulses make a AC voltage, positive, negative,positive,negative. It's these pulses that the speedo counts and by that count it can tell how fast the output shaft is turning and therefore how fast the car is going.
Therefore the two wires to the tranny sensor are the signal. The other wiring to the speedo is a keyed power source and a good ground, and maybe an additional wire for the dash light bulb in the speedo. Hope this is helpful.

[petersond716]

Speedometer calibration, or knowing how fast you're going, can be pretty important. We all know a ticket is about the biggest buzz kill on earth! So if you're about to get bigger tires then stock or already have them installed, its time to calculate your true MPH. Your speedometer is reading how many revolutions your tires are making not how fast they are spinning. When you go to bigger tires the circumference of the tires is larger, meaning the distance around the tire is longer. Thus, with each revolution you are traveling further than before. Therefore your actual speed is higher than your speedometer reads. Or lower if you have gone with a smaller tire then stock. Our speedometer calibration calculator will get you the answer you are looking for. Enter your original tire size, your new tire size, and then play with the miles per hour to see how much faster you are going at different speeds.
If you would like to see more speedometer calibration speeds at a time, visit our tire size comparison page to compare different metrics of tires and get multiple speedometer calibration readings at once.
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[SCFFR]

Mike,

Try connecting one of the wires from the harness (from the trans) to one of the speedo wires and ground the other harness wire. If that doesn't work, connect the same harness wire to the other speedo wire. I have the RF harness and TKO600 trans but with Smith gauges and this is how mine is connected. Basically only using one of the trans harness wires.

Ron

[FFinisher]

One wire from the sensor goes to ground, one wire goes to the yellow wire on the speedo, all the other wires on the guage provide ground for the guage ground for the lights and power to the guage.