Comparing Prices: Local Gas vs. Beltsville Costco Gas

Several of the Montgomery County residents  mentioned that they currently go to the Beltsville Costco to purchase cheap gas, and so they would benefit from a Costco station closer to home.

How cost-effective is it to go to the Beltsville Costco for gas instead of buying fuel at a local (non-Costco) gas station in the Wheaton area?

, a Kensington resident, raised this question and shared some financial calculations . 

Based upon real time data, the cost of going to CostCo in Beltsville is higher than purchasing it in Wheaton...

Cost of Gasoline at local Wheaton station: $3.35/$40.20 to fill a 12 gallon tank, 
Cost of Gasoline at Beltsville CostCo: $3.25./$39.00 to fill a 12 gallon tank, 
Cost Savings of $1.20 to purchase a tank of gasoline at CostCo.

Is there really a savings?

Current Average MPG of American Automobiles is 18 MPG (per http://www.project.org/info.php?recordID=384).

The distance for a round trip from Wheaton to Beltsville is 20 miles. This equates to a cash outlay of $3.61 at the Beltsville CostCo prices to pay for the round trip from Wheaton to Beltsville and back again.

Subtracting the cost savings of the purchase of CostCo's lower grade gasoline ($1.20) from the cost of taking the trip ($3.61) equates to a "loss" of $2.40. Not to mention the time it takes to travel there and back (whatever your time is worth), idling in line, and the burning of fuel while waiting in line. (if a 15 minute wait consumes 1/5 of a gallon of gas while idling, then the cost for that is .65).

Overall loss: $3.05. You've been losing money buying gas at Beltsville.

Increase the cost per gallon another .05 per gallon to use your credit card at another station and you're still ahead.