The code i'm using wont help you, since mine is a massive rewrite.
You want to use "tank.vssPulses", not "tank.vssPulses()". The "()" indicate that it's a function, which it is not; vssPulses is merely a number (unsigned long). You also don't want instant.vssPulses either, as it will reset to 0 every 1/2 second.
In your quote, you also are displaying instantmph() twice. Not a killer, but not useful
edit: probably a typo in your post. if it was actually in your code, it would have produced another error, which you didn't mention.