Drive Fast, Pay the Price

It is a sunny day, the windows rolled down, and no one on the highway. You can’t help but step on the gas cruise a little faster, or perhaps your a few minutes late and want to make up some speed, whatever the reason many Americans find themselves speeding on the highways.

It is no surprise that speeding does consume more gas and in return increases your fuel consumption sending you to the pump more often, but how fast is to fast? What is the optimal speed? How can you go further on the same tank of gas? And, most importantly, how much does your speeding cost you?

What is the optimal speed?

I found a study called the Transportation Energy Data Book that has some very useful information regarding this topic. Unfortunately the data only goes through 2000, but nonetheless it still gives a good picture as to what an estimated optimal speed should be.

First of all, the optimal speed should be defined as the speed at which the least amount of fuel is used to achieve the greatest MPG. Since the engine burns some gas no matter what speed you drive, low speeds will not give you a good MPG. On the flip side, as you increase speed the momentum works to improve efficiency up to a certain point. At that point, the resistance and drag of the car start to work against you and require more and more fuel for just to maintain your speed.

In the study, they sampled 9-cars from 1997 and found that the optimal efficiency occurred at 55 MPH. This increased from an optimal speed of 40 Mph for cars between 1973-1984 (a 37% improvement from car design). It is probably safe to say that the optimal speed in today’s new car designs have inched up a bit. With that assumption the optimal speed of today’s cars probably lies somewhere between 55-65Mph.

So how much does my speeding cost?

In the cars tested, roughly you lose 3% in the first increase of 5mph, and then 7% for every 5 MPH thereafter.

55 = 0%
60 = -3%
65 = -7%
70 = -7%
75 = -7%

The difference between 55 MPH and 75 MPH is -24%. So, if your car gets 30 MPG, your MPG will be reduced to 22.8 MPG driving at 75 Vs. 55. Based on the current gas in my area ($4.79), if I drive 100 miles in a car that gets 30MPG here are the comparisons:

55 MPH

65 MPH

Fuel Used

3.33 Gallons

4.39 Gallons

Cost Of Fuel

$15.95

$21.03

Total Drive Time

1.8 Hrs

1.3 Hrs

So you save 30-minutes in driving time for $5.03 in fuel cost.

One thing to keep in mind, these numbers are based on an average of several types of cars. Depending on your car, you may have to slide the scale left or right. A very aerodynamic car might have an optimal speed of 65 MPH, whereas your H2 Hummer might be 50 MPH. Unfortunately, I could not find any data on specific car models to send you to, but use common sense based on the size and shape of the car.

How can you make your tank go further?

Even if you still do speed, you can make a large difference in your overall MPG by knowing a few facts.

The more you stop the more fuel you burn. Obviously, when you are idling you are getting 0 MPG, but the larger problem comes when you start again. The initial fuel required to move your car is much higher than the fuel required to keep your car moving. Avoid frequent stops if you can.

Similarly, acceleration to quick de-acceleration reduces your MPG. When you are inconsistent with your speeds, your car becomes less efficient since efficiency is achieved when you hold a speed long enough to offset the acceleration.

Some tips:

-Accelerate slower in the city and try to maintain a consistent speed between signals. Speeding to the next light will cause your MPG to decrease.
-On the freeway, try to maintain a consistent speed.
-Avoid roads with lots of stops if possible.
-Don’t prove how fast your car can go from 0-60.
-Remove any objects on your roof.

With any luck, you can achieve 3-15% improvement in your MPG without slowing down.

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