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



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Old 03-28-2010, 10:58 AM   #1  
Adam
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Default 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?
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Old 03-28-2010, 11:13 AM   #2  
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Handheld programmer such as the SCT XCal3
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Old 03-28-2010, 11:13 AM   #3  
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Quote:
Originally Posted by AJ_68 View Post
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?
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.
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Old 03-28-2010, 11:16 AM   #4  
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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.
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Old 03-28-2010, 11:20 AM   #5  
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Quote:
Originally Posted by AJ_68 View Post
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.
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Old 03-28-2010, 11:44 AM   #6  
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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.
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Old 03-28-2010, 03:15 PM   #7  
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+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.
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Old 03-28-2010, 06:25 PM   #8  
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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 . . .
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Old 03-28-2010, 06:57 PM   #9  
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Originally Posted by cidsamuth View Post
By the way, in the picture of the Grabber Blue Mustang, it sure looks like it has 19" rims . . .
18s unfortunately.

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Old 03-29-2010, 05:13 AM   #10  
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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."

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