Conservation for Cash - Driving Slower, Saving Gas

What can you do to save money when gas costs $4.20/gal? Conserve for cash! Driving slower is perhaps the easiest thing anyone can do to save cash and save the nation from itself. Consider the effect of driving on the highway at 60 mph instead of your typical 66 mph; the effect of driving 10% slower will increase your miles per gallon by about 20%, and to decrease the effective cost of gas by about 20% as well. It's like buying gas for $3.80/gal instead of $4.20. Not bad, and not too difficult. You get a similar benefit by driving in the city at 30 mph instead of at 33 mph, but the reasons are different.

On the highway, your mileage is mostly air resistance. Assume that you go typically drive for 1 hr per day at 66 mph and go through 3 gal of gas at $12.60 (that is assume you currently get 22 mpg), driving at 60 mph will save you 20%, or about $2.50, while adding 6 minutes on to your commute. Driving 10% slower is the equivalent of paying yourself $25/hr.

Imagine all the air that your car must push out of the way as you go down the highway. That is a mass with a cross-section that is about as big as your car, and a length as long as your commute. For a typical car making the 66 mile commute above, that's about 10,000,000 cubic feet of air. Now air is rather light, but 10,000,000 cubic feet of it is heavy, about 100 times heavier than the car. You are burning gas to push this gas around your car so you can go through, and the energy to move the gas around is proportional to the speed of the car squared (thus 10% becomes 20%), to the frontal area of the car (drive your can not a minivan if you want to save gas), and to a slipperiness factor.

The slipperiness factor is seen to be quite important, but it is only measured in a wind tunnel by experts, or by consumer reports on a track. It is not reliably indicated by the aerodynamic lines of the vehicle. Sorry. The Volkswagen minibus, for example, looks blocky and inefficient, but it was designed to be very slippery and thus got quite good gas mileage. Most "aerodynamic" looking cars are dreadfully worse. Worse yet you can't change your car's aerodynamics at this stage so the easiest thing you can do on the highway is to drive slower.

Driving slower helps on city streets as well, but for a different reason: starts and stops. For the most part, on the street, you are going so slowly that the air friction is quite low, but you start and stop a lot, and your gasoline energy is lost to your brakes. Since the energy lost this way is proportional to speed squared you again find that your city gas mileage goes up by 20% for every 10% decrease in speed: not bad. For city driving, you can do one other thing too, lighten the car. If you are carrying around sacks of mulch, or other junk, deposit it immediately. If you've got a 4000 lb car with 200 lb. of mulch in the trunk, getting rid of that mulch should save about 5% of your city gas cost, effectively 20¢/gal. Not bad, and this change will make your car safer, and your garden prettier as well. Enjoy.

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