I'm not sure exactly why you think it's overflowing, as I'm showing it on the LCD to be 1400, but here goes...

I haven't put a cro on it (would have to take out out to the car) but, with the 4mhz clock, at idle (roughly 1000rpm) rlow, and rhigh show me roughly 1400 each. So the 4mhz clock gives resolution at 10us, so a complete high/low would be 28mS.