I have recently put a different rear differential in my 2001 Wrx (the old one had damaged gears) and my speedometer is showing that i'm going faster than I actually am. It is as follows:
- When the speedometer shows that I am going 50 km/h, I am actually going 47 km/h.
-But also, when the speedometer is showing that I am going 90 km/h, I am actually going 84 km/h.
*I know this because I have used a speedometer app on my phone to verify. Also, everyone goes noticeably faster than me especially on the freeway and overtake too often.
How do I correct/adjust my speedometer to the correct speeds?

There are a number of ways to do this.

You can get a simple ratio box that will convert the VSS signal to adjust for tyre/final drive differences. I think you can even make your own from a Jaycar kit. Or get from AISAT or online

You can use a device that connects to the ABS wheel speed sensors and outputs a VSS signal which can adjust to match your speeds. DCCDpro make a product called VSSpro that does this. I have previously used one of these and it worked fine. Its made specifically for retro fitting late model boxs without VSS into early model vehicles. But will do what you need it to do.

You can get a box that has a GPS reciever in it and spits out VSS signal. These are pricey, but it gives a good ground speed irrelevant of what tyres you have (so handy if you use a few different sizes of wheels etc.)

You could even use an aftermarket ECU and wire and configer it to do a ratio conversion on your VSS aswell. I do this with my Link ECU.


Also. You say you swapped rear diff ratio? Did you ONLY swap rear diffs? If so then you may have a mis-match between your front and rear diffs and be constantly loading the center diff, which will fuck up very quickly.

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Doing it for all the wagons out there

When I bought the car it already has mis-matched differential ratios. The one I just put in has the correct matching ratios.

Buy a cheap GPS, my Lib is spot on up to 120kph by the GPS but my Transit is about 4-5% out so I just rely on the GPS constantly.

You should be running a 3.9 in an 01 unless it's a 10/01 or later then 4.44.

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Can't remember my old password fucking!