I have a 06 SuperCrew 5.4L 4x4. I ordered the lift, wheels and tires yesterday from 4wheelparts(Superlift 4.5" F.I.T., Diamo DI17 Karat 17x10, and 33x12.5 **** Cepek FCII). The shop installing the lift is telling me that the speedo is based off of the revolutions of the axle so regardless what tire size I run, it will be right??? Is this correct? Just doesn't sound right to me. If not can I take it to Ford to get adjusted or do you have to have Edge or something like that.

 

Yes it is based of revolutions of the axle, but if you put on a larger tire, it will travel a farther distance with one revolution. The speedo can be recalibrated, but it requires an aftermarket programmer or Ford doing it and getting as close as they can with tire sizes listed in their programmers, which is limited.
An example is that a 255/70R17has a circumference of 94.1". A 305/70R17 has a circumference of 106.22". So you travel 1' farther for each revolution. A 13% difference.

 

If the shop told you that and endorses it as truth I'd find another shop, because they are obviously incompetent. Perhaps they midunderstood you or something??
It is as plain as the nose on your face that a larger tire has a greater circumference and therefore rolls farther for one revolution.

On the other hand, going from your stock tires to 33's won't really make a very big difference to your speedo anyway. you'll be off by 2-3 mph at 60 mph. My speedo was a 2-3 mph high from the factory (tested against a radar gun) meaning I was actually going a bit slower than my speedo said, 33's would have probably put it spot on.

 

Check this out;

http://www.web-cars.com/math/tiresize.html

 

Sounds like I will need to take to Ford once it is completed. Any idea how much it costs to have this done? Thanks again for the info. I have never dealt with this with a truck, all of the Mustangs I have had once I regeared the rear I just bought a new speedo gear for the tranny and it was a one bolt deal. So this is new..

 

They changed it a while back to all computer controlled. Prices I have seen from people mentioning this was around $80. Again, they may not have the exact tire or even close in their programmers. They probably would only have stock sized tires.