Im guessing Speedometer uses the GPS to figure out your speed ? That
would explain the battery drain.
A speed condition would have to use multiple triggers to incorporate
but it could be done pretty easily.
The main one is the accelerometer. If that notices a certain G force
for about 60 seconds then it means that you arent standing still,
unless you move very erratically.
The second would be orientation. It would need to be within a small
range of values to allow for some movement. You dont really need to
specify a range if you use a dock or leave your phone on the dashboard
but it would be needed for those who keep their phone's in their
pocket. As far as I know, the accelerometer uses an X Y Z axis to
register what orientation the phone is at.
The last trigger would be a charging/docked condition from your car
charger. This is mainly for people who drive themselves and not for
those who use public transport or cycle.
Im not sure about the accelerometer but these should be pretty battery
conservative as opposed to using the GPS.