I don't know how I did this but how can I change the digital mileage display to read mph instead of kilometers?

Not sure, i've been trying to figure it out but can't seem to find the info anywhere. I assume it will be something like holding down a number of buttons at the same time to make the switch. I guess your best bet would be to call a dealership and ask or check your manual.

I did this on my 1999 996, you need to rotate one of the knobs between the clocks for 5 secs I think. Its described in the owners manual.

On the instrument binnacle there are two knobs that rotate a quarter turn either way. I think it's the one the right you need. Rotate it right and hold it there for around five seconds WITH THE IGNITION ON! It doesn't work if the ignition is off. The digital speedo will then read KmH; repeat to reverse.

thank you and i will give it a try.