This is listed for 94-10 Mustangs but since those years use a VSS for speedometer input (like 85-88 Thunderbirds/Cougars) conceivably this should work on our cars:
https://fordperformanceracingparts.com/parts/part_details.asp?PartKeyField=22362
Instructions: https://fordperformanceracingparts.com/download/instructionsheets/FordInstShtM-4209ADPT-A.pdf
This should work with our cars correct? Yes I know speedometer gears are cheaper but in my case they don't work quite correctly. I have an AOD with an 8 tooth speedometer drive gear. The closest I can get my speedometer to accurate with 3.73 gears is to use a 23 tooth driven gear. It's close but still 1-2mph off. With this adjuster I could use the stock speedometer driven gear (for the 2.73s that came in the car) and have an accurate speedometer.
I can't read the instructions on my phone, but if it is for a vss sensor,it should work fine.
The vss uses pulse width modulation at an "buttstuffog" signal of 8000ppm. I can verify that most 94-98 mustangs did use a vss sensor output. The 98.5 and later oss signal is considered to be a digital signal, putting out somewhere around 32000ppm, and is incompatible with older systems. When i swapped in the 2003 sn-95 t-5, my speed would read 85mph at top of first gear, but otherwise worked fine. According to v6power message board postings, the 94-98.5(with some cross over) did indeed use the 8000ppm signal, like our cars.
More likely the not, it will he a digital conversion box with a divide by function with an adjustable pot. This would explain why it is limited to 10%, but could also work on the newer oss equipped cars.
Basically yes, it would work, but so would replacing the resistor in the dash (for a cost of less then $1), or if you were not going to modify your tire sizes or gear ratios, you could build a bread board setup for probably less then $10, and if you could handle a bit of programming, you could recreate the exact same setup with a simple pic controller, which is probably all that part is for anywhere from $20-80.
But yeah, from my limited understanding, it should work okay.
I'll have to look into building my own. It's probably cheaper than the ~$200 FRPP wants for their box.
Google divide by circuits and the most common setup and easiest with most information would probably be an adrunio.
I wasn't trying to talk ya out of it, but it shouldn't be too hard to duplicate for cheap.
Hmm. I can just borrow an Adrunio from work and see if I can build one of these things.....
Let me know how it works out. My dads a big electronics guru and is using an adrunio to run his homebuilt 3d printer and knows al about that kinda stuff. I was gonna try to build one to make up for the oss and vss differences in my t-5 swap, but i learned that the trans can be retrofitted with the 94-98 style vss stuff with only a tail shaft housing swap and id rather have an oem setup for reliability. Just never got around to it.
Your tires will also be a factor, I've seen more then a 5% difference with the same sizes in different brands and sometimes even within the same manufacturer. Really, a mile an hour or two off really isn't that big of a deal. Unless you went with a constantly adjustable setup, you could end up in the exact same place as where you are now
Either way, good luck, and let me know how it goes.