I just replaced my 17" with the 18" wheels a couple of weeks ago. The dealership changed the wheel/tire size in the computer so the speeometer is still accurate. Total cost: $35 plus tax.
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A user on another board replaced his tires, and Ford charged him $35 to reprogram the speedometer for the new tire size.

You know when I called my dealer and ask them, they told me no way they could change it. And by putting on the bigger tires could change the shift points in the transmission and tear it up. Second dealer told me "no problem". What gives?

Anyone know the CORRECT answer to this?

 

Dont even these high tech trannies still rely on fluid pressure to determine when they shift?? Sounds like bs to me.

 

I'm no automotive technician or engineer but it seems to me if this was the case that you would see trucks of all makes dropping their trannies all over the roads. First dealer was probably too busy, lazy, or didn't know how to do it and tried to lay a load of BS on you hoping you wouldn't know any better. Just my $0.02

 

Go to another dealer! Ford uses what's calld a "WDS" this is our "Computer" to make certain changes, PCM flashes, etc. Takes about 30mins if your vehical has the option they can make the change. Depending on how much bigger you went on the tires will determin the shift points of the tranny. The Tranny is controled by the pcm, which looks at the VSS and various other sensors to determin the amount of pressure to apply and when to shift. So With bigger tires(large overall diameter) you will be going faster than the PCM thinks you are. resulting in earlier shifts. I would how ever consider a gear change if the tire size is substantial enough.

 

I went from 255/65/17 to 275/65/18 about two inchs.

 

That would be enough to change speedo...and as long as the PCM allows the change then it could easily be done.

 

Here's that nifty link once again to see how much the bigger tires should have thrown off the speedo.

Speedo Calculator