Hi solara,

You had specified Timer1, which is slower than Timer0.

If you use Timer0, you can measure up to 50Mhz, even with a 4Mhz osc.

Using the Prescaler, it actualy works like a 16-bit Counter. It's a little tricky to get the lower 8-bits. But it can be done.

Check out AN592 from Microchip
AN592: Frequency Counter Using PIC16C5x

HTH,
   Darrel