Are your result figures correct? It shows the exact same result on the 877A for 6000
and 8000 RPM!

What do you get with something like this on both versions?
Code:
period.lowbyte = CCPR1L		' Store the captured value in
period.highbyte = CCPR1H	' period variable

SEROUT2 LCD,84, [Prefix,CursorPS,20,dec5 period]