If I put larger tires on the truck, what would I have to do to recalibrate the speedometer?

Jim

 

If the tires you installed matched a 'standard' size within Ford's IDS system for yer truck, a dealer could reflash the pcm for you.

If not - an aftermarket tuner would be needed (e.g SCT x3) or something along the lines of a Tirepaq device - which is a lobotomized tuner that only does tire/gearing recals.

MGD

 

OK, if I'm going from a 275/55-18 (standard size) to a 275/65-18 would you have any suggestions? The IDS only has the standard 18", 17" and 15" (I think) in one size of each. The 275/55-18 is 696 or 697 revolutions per mile, depending on brand. The 275/65-18 is 648 R/Mi but the IDS apparently limits the range to greater than 690. (Now that one's gotta make you wonder) This is with a 3.73 gear set according to the computer.

Would it be possible to tell it the truck has a different gear set and make the correction that way? I forget how the math goes, something like 648 divided by 696 times 3.73 gives the ballpark figure for the gears? That gives 3.47 I think, so maybe telling it the axle has 3.31 or 3.56 gears would get the R/Mi in the range and then the formula could be run backwards for the R/Mi? Maybe that's backwards and it would have to be 4.11 gears. If I got lucky and the math is right I think that is around 719 R/Mi which the IDS might accept. The 65's are about 2" taller than the 55's

Jim