I have an 85 Westie 4 speed standard transmission = with 85k miles on it.
On a recent trip from Jasper Alberta to Kamloops BC, = I averaged 19.2 mpg(US) hauling
4 people and all the camping gear plus an extra box = attached at the rear, port a potty etc...
Mountainous terrain through the Rockies, average = speed ~60mph, tire pressure 40psi all around on
Continental tires and using 89 octane gas..  = Did not check anymore for the rest of the trip.
 
Alfred