If you or someone else has not chnaged the transmission or rear end, you should be in the ball park. Also if the rear has been changed and you know what rear ratio you have in your truck you could probably find the right speedometer drive gear.

Michelle

 

I am going to have a similar problem once I get mine running with my big wheels.

I'll have a 2 speed axle also, so using a conventional driveshaft/trans driven cable or pulse counter etc just won't work. (Well, it will work but every time you use low range or shift the rear axle the thing will be way off!)


So, I'll use a VDO programmable speedo. It will count pulses from 500 to something like 400,000 pulses and then display it after you travel a measured mile. (You press the calibrate button after traveling 1 mile)
I plan to use a Hall Effect sensor on one of the front wheels and epoxy enough little magnets on the sensor plate to give me at least 500 pulses in a mile. (Actually I believe the speedo will be more accurate if there's more pulses per mile so maybe I'll use 4 magnets on the plate....)

To make it work with our old trucks one has to fabricate a device similar to the speed sensors found in modern vehicles with anti lock brake systems. Each wheel has a wheel speed sensor that basically counts pulses generated by the sensors. These could be either magnetic pulses or light pulses. Magnetic pulses are better since they are not usually affected by mud and/or debris on the sensor. Light pulse sensors have to be sealed.

Magnetic sensors simply detect a little magnet passing by a conductor. They are also known as "Hall Effect" sensors.

Lookee here>> http://en.wikipedia.org/wiki/Hall_effect_sensor for definitions and an animation of the concept.

I'm a "Closet Wrench Turner" too but I also happen have an electrical engineering degree.....I never used it for "real" though other than for my own hobby use........I just like to tinker with magic!


Cheers,

Rick