Kind of slapped together
Join Date: Jan 2010
Location: Bossier City, LA
Here's a wild idea.
For under $10, if you have the time for ship-from-China, you can order a function generator kit.
Then, doing the math ( 8,000 pulses per mile, 60 seconds in a minute, so 133.333 pulses/second = 60 MPH - use other frequencies and solve the other way for way points on the speedo! ) you can adjust the needle on the bench, no "Let me plug this in while doing 60MPH in traffic!"
Takes a 9V battery or so for the function generator; use a full 7805 instead of the small 78L05 the kit comes with, heat sink it, and you can drive it, and the speedo, off a 12V power supply or battery.
I find this a LOT easier.
Oh, and crank it to about 260Hz on the function generator, and you can get about 120MPH to bring the odometer up to the proper setting *grins* Just be aware that it takes about 8.5 hours to run 1,000 miles, so about 3,000 every day approx; if you're about 120,000 off that's an easy 45 days of it just sitting there running.