Kman,

You almost had it.
Just need to move the GOTO Main down 1 line, and clear the timer when it reaches 100 instead of after you Print it to the LCD/Terminal.
Code:
Main:
    SerOut2 SO,16468,["Milage = ",DEC Miles,".", DEC Tenths ,13]

Loop:
    VSSCount = TMR0 		 ' Get TMR0 count  

    IF VSSCount > 99 Then 
        VSSCount = 0
        TMR0 = 0
        Tenths = Tenths + 1
	
        IF Tenths > 9 Then
            Tenths = 0
            Miles = Miles + 1
        EndIF
        GoTo Main
    EndIF
GoTo Loop
OR, you can reduce it a little so that it works with the timer directly. Get's rid of VSSCount.
Code:
Main:
    SerOut2 SO,16468,["Milage = ",DEC Miles,".", DEC Tenths ,13]

Loop:
    IF TMR0 > 99 Then 
        TMR0 = 0
        Tenths = Tenths + 1
	
        IF Tenths > 9 Then
            Tenths = 0
            Miles = Miles + 1
        EndIF
        GoTo Main
    EndIF
GoTo Loop
Now you just need a way to save it to EEPROM, so you don't lose the value when power is lost.

HTH,