I have no idea how it's actually done. I got a speeding ticket 13 years ago and I remember that I was able to get out of it by taking it to a place for recalibration. Gave me a paper to take to court and everything. I'm assuming that it's possible to do since kits are available online.
The FX is not like standard domestic trucks which can have the speedo recalibrated with a flash tuner.
As far as I can tell no G35, or 350z has ever been able to get their speedo adjusted, everyone just seems to say it is impossible.
HOWEVER, the titan guys have tested multiple methods, and although very few have actually bothered doing it, it appears that hypertech has finalyl developed an inline speedo calibration adjuster which will work with the titan, and will HOPEFULLY work with our vehicles as well.
Discussion here:
http://www.titantalk.com/forums/titan-performance-modifications/106804-calibrate-speedometer.html
product here:
http://www.hypertech-inc.com/products-speedometer-calibrator.aspx
Keep in mind once again, that you are going into uncharted waters, there is currently no FX35 owners who have successfully made this work, and I almost dare say there is no 350z or g35 owners either.
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On that note I already have the tool inbound to try it out on my 2008 FX35, my new tires are going to be roughly 2" shorter than stock (i'm going from a 30.4" diameter, to a 28.1" diameter). I was going to update everyone if it works once I actually get a chance to try it out. At $200, its not cheap, but if it prevents my odometer from climbing at like a 10% higher rate then it is worth it.
---------- Post added at 08:45 AM ---------- Previous post was at 08:42 AM ----------
I have never heard of getting your speedo recalibrated. I don't even think that's even possible
It is super standard for guys with lifted trucks, cause they go from 30" tires to 40"+ tires depending on how aggressive they are going, so at that point the difference is shocking to both the speedometer in your car as well as anything which uses distance calculations, so fuel economy readouts, distance to empty readouts, your Odometer, trip distance, etc.
Can get pretty annoying when you have only gone 10 miles, but your stupid trip calculator says you have only gone like 6.