Trip cost from distance, MPG and price/gal.
Fuel cost for a trip = (miles ÷ MPG) × price per gallon.
Simple, no adjustments for terrain or driving style — those can move the number ±10–20%.
This is a useful baseline for comparing trip costs across different vehicles or routes, even though the real number will drift somewhat with traffic, elevation changes, and how aggressively you drive.
300 miles ÷ 28 MPG × $3.50/gal = $37.50.
Use the closer number to your trip. Combined works for mixed routes.
No — this is fuel only. Add tolls separately for total trip cost.
Meaningfully — aerodynamic drag increases sharply above about 65 mph, so a trip driven faster can burn noticeably more fuel per mile than the same trip at a moderate highway speed.