what tranny stock or after market, i,ve got the 3550 w/23tooth and my deputy sherrif buddy paced me down his road and my speedo was right on, those crown vics have certified calibrated speedos so i took his word on it.
well i had a friend drive next to me with a walkie talkie when i got my gear install last year. i have the white tooth gear (think thats 23). with the stock auto and the t-5 in there now, i am 1mph off for every 10mph i go. works all the way up to 100mph too.
the mph should be more off the faster you go and there is no set amount of mph that you will be off.
I didn't have a chip. The 2-3 shift was really bad. I just got the chip, so I'll find out this weekend what the 4.10's really did (if the rain stays away ).