why does the calibration of the speedo make a difference? how do you calibrate a non digital speedo? is it hard?

 

It doesn't matter unless your speeometer is electronic, not mechanical. If electronic (92-96 trucks and newer) it sends signals to the computer which monitors and adjusts shift points in electonic trannys (E4OD). If mechanical, you just get the wrong speed and your odometer will be off.

 

[updated:LAST EDITED ON 15-Nov-01 AT 05:29 PM (EST)]To calibrate a mechanical speedo you change the speedo gear in the t-case. It's super easy, on my '90 you need an 11mm (or maybe 3/8"?) socket to pull one nut. Then the cable and speedo gear will pull out. Pop off the c-clip which retains the gear, slide it off and slide the new one on. Install is reverse of removal (do I sound like a Haynes manual or what?) :-)
You can get the speedo gears at your friendly -or not- Ford dealer for around $12 a pop. Some of the more common tooth counts are 17, 18 and 19, IIRC they go as high as 21 or 22, not sure about the low side. Also, each tooth count is color coded, I think white is 17, black is 19, and 18 might be yellow..... don't quote me on any of that, tho.
Later models w/electronic speedos get their reading from the excitor gear for the abs in the 8.8. To calibrate these (only necessary if u change tire size as a gear change alone will not affect the excitor ring) you have to ..... pull the instrument cluster... I think?... and push some buttons or something. Sorry for the technical terminology, I've never done it but I know there's several links out there explaining the process. I do recall you can only re-calibrate them six times.