An issue that troubled me since, well, I started paying for gas is what is the optimal speed for saving money. To put my mind as ease I asked some knowledgeable people in the car field, that got me some conflicting answers. So I read a bit, thought a bit and got my answer.

As the car drag (which is directly influenced by the car speed) increases in a rate bigger than that of the speed, it is quit clear that the faster you go the worse your fuel economy is. However, a stopped car (engine running) has the worst fuel economy as it covers no distance and use gas. And so we come to the conclusion that there has to be a speed somewhere in the middle with the best fuel economy.

To make things clearer we can say that we want to go as fast as possible for as little gas as possible – this is the key to solve our dilemma! but first some things about gears.

Gears translate, correlate and pass the engine speed to the wheels. We all see and know that if we drive in 2nd gear and shift (or the automatic system shifts) to 3rd we see a decrease in the engine RPM. Driving in higher gear makes us spend less energy for the same speed – the ratio between engine and wheel speed changes with gear – that is the meaning of gears…

And so we come to the conclusion that the best speed has to be the one you find yourself shortly after shifting to your highest gear. Yes, for each car and model this speed whould be different but it is safe to say it will be at around 45 MPH/70 KPH.

Please note that this analysts is right for cruising, as driving in the highest gear at a relatively slow speed means that if you need to accelerate foe some reason you will need to step on the gas quite a bit and probably be better off going one gear lower for the acceleration gas wise.

How much money will I pay if I drive over this theoretical speed you ask? Well, that depends on your car but to get some notion you can check out this graph from EPA (the US gov is so good at this staff…), Unfortunately it doesn’t show data below 40 MPH.

Speed Vs MPG – EPA

And so, we can lay to rest ideas such as “drive as slowly as possible” or “keep the speed needle in the vertical position” and start saving gas, or at least know what we do that spends it so fast…