Ok so, my cars (subaru engine subaru transmission) electronic speedo is double the actual vehicle speed. The Vehicle Speed Sensor produces an AC voltage and from my understanding the ecu reads the frequency of the voltage (if it reads the output voltage, correct me). How could i reduce the frequency by half so my speedo works more a less correctly?
-its not to do with the speedo im going on the number on my cobb access port which is attached to the obd 2 port

https://www.amazon.com/Speedometer-Odometer...ps+speedometers

I would but i want my ecu to read tge right speed

So the engine and transmission and the speedo are from the same car but now the speedo reads double?

What engine computer and speedo are you using?

If stock Subaru engine computer, leave that alone as it will want to see a proper relationship between engine RPM, and road speed

If stock Subaru speedo, everything should be working fine.

I'm wondering if you are looking at KM / H vs MPH being 1 KM = 0.6 Mile

Well kinda, its a different read end: 4.33 vs the 3.9 the wheels are also a different size. i don't know if the math works out but 60 is 30 and 130ish is about 60 to me thats about 200% increase
It could also b that the wrx uses a different vss than the legacy that outputs a different frequency or a different magnetic ring

Where is the VSS mounted? If it is reading from the axle flange area or a toothed wheel on the diff ring gear, actual final drive ratio is meaningless since it is reading wheel revolutions.

What size is the stock Subaru tire and what are you running now?

Get better at dividing by 0.5 ??

It's located on the r side of the transmission. That being said; mine is connected to the ECU but I have a GPS speedo. If I disconnect the sender the car runs fine but at constant speeds i.e. the hi way, it will quit about every 8 miles until you move the accelerator pedal then it's normal again.

That would double it... <---math pedant. Also hate when someone describes something smaller as, for example, "5 times smaller"...

OP, what's the speedo? Many have options for tuning by adjusting the frequency input. Barring that, there's no easy way to adjust frequency input without inserting some kind of electronic frequency adjuster in between the output and speedo. How to do that would depend on the systems you're using.

You know what the speed error is. Don't sweat the small stuff at this point.

Click to view attachment

I think the easiest to fix your issue and for 100 dollars, get a speedohealer. It accepts all input signals, has a microprocessor to convert to the output to what you need.

Look at Tach-Adapt

If you want to go more "homebuild" you can build a frequency divider circuit using a D-type flip flop counter assuming the sensor is hall effect (square wave). If its a VR sensor you'd need some additional processing.

What has been suggested in the 2 previous posts are the answers. Well, Kent too.

Kent, was that the fix for the issue that was dogging you during rt66?

Thats what i was thinking, thanks for all the input everyone

So the speedo is GPS but you want to alter the VSS that apparently isn't connected to the speedo to correct 2X speed? That won't work.

Do you have a set up sheet for the GPS speedo?

Idk about him but i have no speedo gauge rn (therefor no gps). I am reading my vehicle speed off my cobb access port which is attached to the obd2 port which tells me the ecu thinks the vehicle speed is twice what it is

You need the suby speedo to convert the vss signal to what the ecu needs to see.

OK got it, I didn't look at the name to see it was not you.

The assumptions for the system you have are as follows,

Subaru trans that reads from the axle stub or toothed wheel on the final drive.

Subaru ECU wiring harness.

VSS plugged into the Subaru harness.

Subaru ECU with a Cobb reader attached

Some unknown speedo that apparently isn't Subaru.
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I'm voting that the reader is showing you KM / H not MPH, are your coolant temps in F? If not, more possibilities are below.

If the VSS reads from the stub axle shaft , or a toothed wheel on the final drive ring gear, final drive ratio won't matter.

For tire size to cause 2x readings, the current ones would need to have 1/2 the circumference of the Subaru. What are the stock Subaru and current tire sizes?

If the trans isn't from the same car as the ECU, perhaps there was a design change along the way doubling VSS tooth count. A low tech way to fix that would be to remove every other tooth.

If this ECU has a speed limiter, you should hit fuel cutoff at half of regulated speed. Has that occurred yet?

Make a new face for the speedo with all the numbers divided by 2?

Sharpie overlay on the speedo.

Seriously, check the signal type. If it's pwm (pulse width modulation) you can get a pulse width modulator to tune it to the correct speed.

Ok, Ok, it was going on 11. Multiply by 0.5