If I go from OEM 18" wheels to OEM 19"s how do I fix the speedometer?
 

Title says it all. Thinking about swapping my OEM 18s for OEM 19s. Do I have to buy an expensive programmer or can the dealer do this for me at a relatively low cost?

Handheld programmer such as the SCT XCal3

if the overall diameter is the same after the swap you don;t need to do anything. There is a line somewhere around here to a calculator that tells you if your speedo will be off, but i can;t find the thread. I bet you won;t need to do anything.

18" tires: 235/50/18
19" tires: 245/45/19

According to this tire size calculator the 19" wheels will register 1.6% too slow. This may vary due to the actual diameter of the specific tires. I guess it's pretty negligible.

Yep, at 1.6% too slow i woudln;t even bother.

Being within +/-2% is essentially nothing.

I just checked mine from swapping over to the OEM 2007 GT500 wheels and tires, and I'm going 1% too fast. So at 60mph, I'm actually going 59.4mph. That's not even noticeable enough to justify changing.

+1 on the tuner. It will allow you to make revs per mile adjustments in increments of 5. I used a GPS and cruise control to get mine dialed in. Every speed sign I see now I use to make a check. I am spot on.

Another way is to look up the rev's per mile value for the size and brand tire and put the closest value in.

I, too, went from the 18" to the 19" OEM rims. I didn't bother with the speedometer, it was that close.

Eventually, I moved up to 4.10 gears and had to adjust the speedo. I bought the tuner -- was just the excuse I needed :)

By the way, in the picture of the Grabber Blue Mustang, it sure looks like it has 19" rims . . .

18s unfortunately.

http://zippo.sector2.com/pics/10mustang.jpg

Okay, the 18s are more obvious in the larger picture.

I definitely recommend the upgrade. I really knew it was money well spent when my wife said, "I can't tell the difference from the old ones."