Three numbers set the cost
Fuel cost comes down to how far you go, how thirsty your vehicle is, and what fuel costs where you fill up. Improving any one of them lowers the total: a shorter route, a more efficient vehicle, or cheaper fuel all reduce what you spend on a trip.
Planning a road trip budget
Estimating fuel ahead of time helps you compare routes and vehicles and set a realistic travel budget. Add a small margin for detours, traffic and price differences between stations, and remember that a fully loaded vehicle or roof rack lowers economy and raises the cost.