Speedometer correction

[TopNotch]

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:

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.

Here is a closeup of the corrector:

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.

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.

[Fierofool]

That's very interesting.  Not only are cars like ours with different drivetrains running around with inaccurate speedometers, but even new cars.  When my daughter bought her first new Santa Fe, she discovered that the speedometer was off by 10%.  70 MPH was actually only about 63 MPH.  The car had a 100K mile warranty, so she would have effectively only gotten 90K miles under the warranty.  When we brought this to the attention of Rick Case Hyundai of Gwinnett, they told her to just put bigger tires on it when she replaced them. 

I have used my GPS as a reference for my actual speed, but I don't always transfer it from one vehicle to another.  Is your device accurate across the full scale?  I find that my 87 is accurate at 70 MPH but reads higher and is more inaccurate as speeds decrease.

If that could be hidden on a new car, calibrated to read 50% of true speed so you could mentally calculate your true speed, you could get 200K under the new car warranty.  Of course, it would need to be reset before having it dealer serviced. 

Would that also work with the Fiero ECM's?  How does it affect the comparative tables in the ECM.  Tables like throttle position and RPM that are compared to VSS? You might want to seek a patent on that. 

[TopNotch]

In a perfect world, the speedometer's reaction to the pulses it receives should be linear. So if a certain number of pulses per second causes a reading of 50 miles per hour, then twice that many pulses per second should cause a reading of 100 miles per hour. The Fiero speedometer seems to be pretty good in that respect.
If a linear speedometer reads 10% fast, it will read 55 MPH at 50, and 110 at 100. The corrector applies a ratio that corrects the percentage that the speedometer is off. So it will correct the 55 reading back to 50, and the 110 reading back to 100.
If the speedometer is not linear, then all bets are off.

[ron768]

Strange, when my 85 was new, I had the speedo checked for accuracy. According to the dealer, it was dead on as delivered from the factory. ( all stock, 14 in wheels). Today , I know it reads slow due to running the 15 in lace wheels or the 16 in spoke wheels that I have.

[HarryT]

I always have a GPS on the dash and the speeedometer agrees almost perfectly with the GPS at all speeds.  I am running stock wheels and door plate size tires.
Harry

[Fierofool]

I don't remember whether or not any of my cars were accurate when they had stock wheels.  I always go up 1 size on tires whenever i replace the stock tires. 

My 87 was running at 95% at 70 MPH with 225/60/R/15's on the rear, and that was with a VSS gear correction.  When I had the 7730 ECM programmed, the accuracy of the speedometer was one of the items in the tuning specs.  I gave tire size, GPS at 70 MPH and the speedometer reading.  Sinister Performance programmed the chip to correct the variation, and now the GPS and speedometer agree.  If I should ever change tire sizes in the future, the chip can be reprogrammed to accommodate them.   

[SpadeCustoms]

I have a Dakota Digital in my 4.9/F40 combo. Building a correction device is more of a challenge without Radio Shack anymore. The Dakota is a very nice piece that allows you to fine tune with your phone so you can match the GPS speed on the go.

https://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=1192/mode=prod/prd1192.htm