Hey guys, I have been dealing with my speedometer being off by 4-5 MPH and I had my tires/wheels and gears changed a month ago. 16"-->17" and 3.27---> 3.73 The mechanic that tuned it for me said it is okay to run it like that but it irritates me. I have a GPS I use and that tells me approx. that I am about 4-5 MPH off. I dont know how to use the Ford Racing Pro Cal tool. Would anyone care to teach me sometime? would be great if we could even fix it all together.