Gas Saving Tip: Slow Down

Your car requires gas to power the engine.  The more power the engine requires the more gas you have to give it.  Pretty simple so far, right?

You’ve probably heard someone say “today’s cars are designed to go 70mph”, and that would be correct.  However that doesn’t mean cars are designed to get their best mileage at 70mph.  To the contrary, they do not.  Let’s get into some math (don’t worry, pictures will be provided).

The average vehicle falls within a reasonable distance of this formula:

Driving Load = av + bv²   + cv³

…where av + bv² covers how much power it takes to overcome mechanical resistance (bearings, joints, pulleys), friction (tires on the road), etc. The last part, cv³, covers the drag, or wind resistance.  As you can see, the faster you go the more there is for your car’s engine to overcome. Av, bv, and cv are all equal to the speed you’re driving.  Graphed it looks like this:

Going from 50mph to 70mph requires 173% more energy to gain 40% more speed (most cars will be in their top gear by the time they reach a sustained 50mph speed).  If you have a twenty mile commute that is entirely highway miles that extra 173% energy will gain you around seven minutes.  Let’s consider, however, that your commute is a more realistic 60% highway, 40% city.  That means that only twelve miles of you commute is highway, meaning you will save a whopping four minutes.

Those of your who are mathematicians, auto mechanics, physicists, and all-around skeptics will be ready to point out some missing variables.  Different engines operate more efficiently at different RPMs, for instance.  So let’s go to the source that we all pay for with our taxes: http://www.fueleconomy.gov/.  According to their site:

While each vehicle reaches its optimal fuel economy at a different speed (or range of speeds), gas mileage usually decreases rapidly at speeds above 60 mph.

You can assume that each 5 mph you drive over 60 mph is like paying an additional $0.30 per gallon for gas.

As we can see from the chart above the average car gets it’s best miles per gallon between 55 and 60mph.

So please, slow down. You’ll use less gas. You’ll probably also arrive at your destination much less agitated about that guy that cut you off, that guy that was only going 80mph in the far left lane, or whatever your pet peeve is.