TinyGPS and SPI

Hello,

is there any way how to get the tinyGPS library work with my andtx GPS shield :

It uses a four-wire SPI interface, however, tinyGPS only lets you choose 2 pins (RX and TX).

Any help appreciated.
What I basically need is the gps-Speed function, i.e., how to calculate the speed of the
device from consecutively received gps coordinates.

THX