speedometer calibration

[J_Persons]

Thinking about changing to an 18" wheel and tire, but the speedo will need to be re-calibrated.  Anyone know the a procedure for this?

[AJP turbo]

I won't have to do anything at all if u keep the overall tire diameter the same or within about .2"...which is what I would do.....or if u must change far from stock only a tune can make the change...I'll look when I get home but u will need revs per mile

[SHOdded]

http://www.gordon-glasgow.org/tirecalc.html

[AJP turbo]

Only problem with those calculators is that 2 tires of the same dimension can have different revs per mile . The tire mfg will test and certify the tire and the revs per mile changes due to loading and flex of sidewall and makeup and design

[SHOdded]

Those factors can definitely change tire height, so can tire wear.  But shouldn't we be worried about all these factors only if we are calculating precision to the nth decimal?  The only reasonably accurate way to calculate rev per mile otherwise would be to compare a new OEM tire with a new "aftermarket" tire on the car, loaded or not.

[jmr061]

If you pick an any OEM tire size available on the Taurus/PI Sedan a dealer can change it with their IDS software.  Takes 10 minutes.