Some of the walking/car units limit the maximum speed and if you fly a reasonably fast aircraft they don't work.
I would suggest looking for something like a Garmin pilot III on ebay. Excellent unit with good battery life on AA's.
You can get a car kit for some of the Garmin aviation units. I tried one and it wasn't very easy to use in a car. Best get a TomTom for the car, you can get one for about the cost of the Garmin car kit.