For car-sickness, we give our 15 pound dogs:
1/4 tablet Dramamine or generic motion sickness tablets 50 mg tablet(dimenhydrinate/antiemetic)
12.5 mg of medication.
Cream cheese is a great way to encourage the dog to ingest the imbedded medication.
For 30-40 pound dogs: 1/2 tablet
An easy way to cut the tablets:
Sharp paring knife or single edged razor blade