I use a dashboard mountable Garmin GPS V and it's plenty for me on the highway. The unit is for driving and one of the options is for the unit to be biased to a road (assume it's on a road) which gives it an additional piece of info. Setting it up as off-road works, but not as well. Usually though I'm using it as an ETA and mileage calculator. I know I'm on I-5, how far 'til my next gas stop and when will I get there. It does those calculations much better (more accurately) than I can and it's continually updating.

My primary GPS for hiking is a non-mapping and very small Garmin Geko 301. I use it for marking my car's parking spot, landmarks on the trail and (more importantly) major landmarks locations off the map. Preflight includes getting the GPS and map on the same page. GPS batteries last a long time when you only use it for the occasional fix and otherwise leave it off.

Neither unit has SiRF technology, not really needed for what I do.