some gps help

[Doug Sawyer]

Trying to trolling for some gills. It is suggested to troll at speeds as slow as .8mph. Well my current garmin III gps  reads as slow as 1.0mph and anything slower than that it reads 0.0 mph. I've been getting hits at speeds under 1.0mph. To be consistence I need to know my speed.

So what I'm asking is does anybody know of a gps that will read as slow as 0.8mph and may be slower.

I'm not interested in a high dollar fish finder or Luhr Jensen speed indicator.

I'm hoping someone can give me a make and model of a gps that will read may be as low as 0.7 or 0.6mph