Just for comparisons sake.
We just made a trip to Yellowstone ourselves, from the east coast, in our
stock 1.9 l  '84 Westy but with no canoe.
We did 5600 plus miles.
Crossing Kansas and South Dakota at 75 mph plus with a/c we were getting 18.8
mpg.
When my wife drove, a little more gently, we got about19.5-20 mpg.
In the parks,toodling around, we got as much as 23.
If you get 18 mpg driving fairly hard,with city driving,you may not be off
much at all.
I have the timing set at about 5 degrees BTDC instead of the book 5 degrees
ATDC.
Otherwise stock.
Hope this helps.
Steve Cotsford