Only reason i ask is because the FX4 i just baught has got 285/70/17 mud terrains on it, obviously much larger than the stock size. My buddy told me to be careful and watch me speed because the speedo would be off 5-8mph or so. I used my GPS today and the speedo is almost dead on. How might the previous owner had this re-caled? Im almost wondering if the truck has a chip, since its got a nice JLT intake, and a flowmaster muffler. The truck is very stout and shifts nice and firm with no lag.

any ideas?

 

Either they put a tuner on it or had the dealer reprogram the tire size.

 

You can get the Superchips Tirepaq Speedometer Programmer for about $200 and do it yourself (it can also read and clear engine codes). I think Hypertech makes one as well, for a little more $$$.