Hi, new member here. I just bought a 1999 Mercury Villager with
the 3.0 engine. I am going to be using it as a mail delivery rig,
and need to put a slightly larger tire size on for puncture
resistance. Does anyone know of a way that I can recalibrate the
speedometer to be correct? I called the dealer, they say the 99
can't be reprogrammed. Thanks.

Well from what I know thats a OBDII system. Which you should be able to reprogram the computer via. certain scan tools. Autozone sells one for around $250.

How much bigger are you going? If you're only going up a little bit, it's easy to figure out how far off your speedo is, and do the math on the fly. I'm over by 3% from stock, so at 100km/h indicated, I'm actually traveling 103km/h. Here's an easy calculator to help you figure out your % difference.