The speedometer in my 3800-powered Fiero reads high. Since I'd like to know how fast I'm really going, and how many miles per gallon I'm getting, I decided to see if I could correct it. I did a little searching, and found something called a Widget Man Universal Speedometer Corrector. It takes as an input the pulses from your VSS or ECM and adds or subtracts pulses as needed to correct your speedometer. I did a little testing and found that in my case, it had to be inserted into the data stream before the correction circuit I had to add to make my Fiero speedometer work with a 3800. The correction circuit looks like this:
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That circuit is located behind the glove box console, so I mounted the corrector behind the passenger seat, where, if you will recall, is where the PCM (Powertrain Control Module) is. I stuck the corrector to the carpet with Velcro.
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Here is a closeup of the corrector:
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The two little push buttons are used to adjust the correction ratio. A number greater than 1 makes your speedometer read higher, and a number less than one makes it read lower. When you push the buttons, the display lights up briefly to show the current ratio.
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I set the ratio to 0.900 and went on a little test drive, with a speedometer app running on my phone. That's pretty close to the ideal setting -- my speedometer agreed pretty close to the phone app. I'll have to do some highway speeds for a better comparison.