[McTimson]

It could easily be done, but you need to make sure your odometer is accurate before you do it, or you'll have a tougher time calibrating it.

Lines 263-274, and 532-565 define the different screens, all you have to do is change something like this:
(line 267)

  doDisplayBigInstant,

(line 540)

void doDisplayBigInstant() {bigNum(instantmpg(),"INST","MPG ");}

to this:

void doDisplayBigInstant() {bigNum(instantmph(),"INST","MPH ");}

And line 267 could remain the same.

You could add another screen, but it might increase the sketch size too much, so it might not fit on the ordinary board.

Changing how the stuff is displayed is easy, if you root through the code around those line numbers, you should be able to figure out how it works. You can use something like Notepad++ to show line numbers if you don't have a program to do it already.