One of the core concepts behind Automatic is that small changes to your driving habits can have a huge impact on fuel efficiency. A few weeks ago, we analyzed our users’ highway driving data and discovered that slowing down just a little could save hundreds on gas per year.
This week we follow up with another aggressive driving habit Automatic helps you with – hard accelerations: what they are and why they hurt your fuel efficiency.
What’s a hard acceleration?
At Automatic, we consider an acceleration “hard” if you speed up 7 MPH or more in one second, equivalent to going from 0 to 60 MPH in about 9 seconds. In most cars, you have to floor the gas pedal to do this.
Your car usually tries to maintain the most efficient ratio of air and fuel in the engine to deliver high fuel economy. But if you hit pedal to the metal, even efficient cars interpret that as a demand for maximum power. Your car chucks that efficient air-fuel ratio out the window and dumps fuel into the engine. Your engine RPM spikes and you feel a surge of power– but you end up burning much more gas than you’d need to reach the same speed more gradually.
Why does it matter?
How hard you accelerate is especially important for city driving. Unlike highway driving, which is usually pretty steady, city driving requires frequent speeding up and slowing down.
You can see this in a plot of speed over time for one of my recent morning commutes through San Francisco. I traveled nearly half the total distance accelerating. For this kind of driving, accelerating gently has a huge impact on fuel economy.
By comparison, here’s the plot of trip I took from the suburbs, over the freeway, back to San Francisco.
So how bad is hard accelerating?
The image below shows how the MPGs vary for a BMW 328i at different speeds and accelerations. Red areas are extremely inefficient.
Based on aggregate fuel consumption data from dozens of 2009-2013 BMW 328is.
In the last post we explored the effect of speed on fuel efficiency (spoiler: driving over 70 MPH is really inefficient). But no matter your speed, accelerating hard wastes a lot of gas.
Deciding exactly how fast to accelerate is a matter of personal choice, but it’s amazing how inefficient it is to slam on the gas. So if you want to save some money, listen when Automatic chirps and lighten up on the pedal!