A trip estimate becomes more useful when it compares real choices instead of producing one isolated number. Run each reasonable route with its own miles, likely MPG, diesel price, tolls, and idle time. Use the same cost categories for both so the comparison stays fair.
Compare the route, not just the mileage
Route A is 430 miles with no tolls. At 7.2 MPG and $3.70 diesel, it needs about 59.7 gallons and costs about $221 in fuel. Route B is 402 miles with $32 in tolls. If the easier road holds 7.6 MPG, it needs about 52.9 gallons and costs about $196 in fuel, or $228 after tolls. In that example the shorter route costs roughly $7 more, so time, parking, and traffic may decide which one makes sense.
MPG assumptions can flip the result. A route that looks cheaper at 7.5 MPG may lose that advantage in hills, stop-and-go traffic, a headwind, or winter weather. Run a second estimate at a slightly worse MPG to see how sensitive the plan is before treating a small difference as meaningful.
Check the fuel-stop price
Use the price you can realistically buy along the route. On 70 gallons, a 20-cent price difference changes the fuel bill by $14. That may justify a different fuel stop, but extra miles, idle time, access, and carrier fuel policy can erase the savings.
Idle burn remains equipment-specific. The DOE methodology below uses 0.8 gallon per hour as a planning value for a heavy-duty diesel; replace it with a measured rate when possible. Fuel and entered trip expenses are still not the same as total operating cost.