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The Hidden Costs of Aggressive Driving

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.

acceleration graphs.001

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.

acceleration graphs.002

By comparison, here’s the plot of trip I took from the suburbs, over the freeway, back to San Francisco.

acceleration graphs.003

On that trip, 90% of the distance was covered on the highway, so my choice of highway speed had the largest impact on fuel efficiency.

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.

acceleration graphs.004

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!

  17 comments

  1. Travis   •  

    Please give us Automatic users access to this sort of data for our own vehicles. Thanks.

    • Nick   •  

      I agree! I would love to have access to this type of data!

      • Mo   •  

        Any data? Sure wish it supplied data on a regular basis.

        Returning it since I find it to be useless. Something that works only 50% of the time I would not pay 100% for.

    • Ryan   •  

      At the very least, give us API access initially. I don’t need pretty data, just *any* data.

  2. Mike B.   •  

    I third that notion! :)

  3. Kyle Shay   •  

    Hi Ted

    Have been waiting for something like this. Bought one. Can’t connect to the unit and and get a quick flashing light. My car is an HHR though purchased in Canada. Is that the issue? If so let me beta test the Canadian connection!

  4. Andrew   •  

    I own a manual transmission 1998 Honda Accord. I’ve noticed some inconsistencies while accelerating. Automatic is supposed to chirp when accelerating more than 7 MPH/s. To keep Automatic from chirping, I accelerate slowly. Even the Prius drivers accelerate considerably faster than me.

    In order to accelerate slowly, I either give the car less gas and/or release the clutch slower. To prevent Automatic from chirping, I attempt to go slowly but my car has so little power that it almost stall, lurches violently, Automatic chips anyways. When I am driving other people I purposely ignore the chirps and drive harder so my passenger(s) have a smooth trip.

    Does the data Automatic uses to determine acceleration come from the car or from an on-board sensor? I would guess that Automatic uses an accelerometer, which explains why it chirps even when I’m accelerating very slowly. Is it possible to use the data from the car instead? I believe this data would be more accurate, allowing me to give my car a more power allowing for smooth accelerations without Automatic chirping.

  5. william thompson   •  

    Conceptually this seems logical but the author is limiting real world impacts to actual MPG which are outside of the drivers control, namely weather and secondly EPA rules which affect the gasoline mix seasonally. I would agree on the city chart impacting rapid acceleration, but occasional rapid acceleration will not impact the overall MPG, also the model, engine size of the car will weigh in on the metric set. Having an underpowered car with a 0 to 60 time of 9 or 10 seconds or longer will under acceleration use more gas. A car which can do 0 to 60 in 5 seconds or less will accelerate faster than the 7 mph per second noted and most likely not change the overall MPG per tank if these accelerations are occasional vs. frequent. Like dieting, its all in the portions.

    I’d rather see metrics matching the specific car vs. one size fits all. Big V8 would need to fit a different profile than a 6 or 4, turbos change the profile as well.

    As for speeding over 70, on the east coast, you need to keep up with traffic and frankly 75 or 80 should be the threshold or user configurable. A short stint over 70 will not impact your overall MPG again, it’s back to portions. What I’d rather see is a metric based not just on exceeding 70 but weighed to the overall tank or trip.

    I like the product minus the reliability issues which are either tied to a good connection for the device or upgrades which occur where the application needs to be restarted, usually noticed after I’ve missed a trip.

    Keep working on the product, but the real value may be in extensions and showing more detailed graphs between like model cars and trucks vs. me against a bunch of Prius drivers in sunny California….

  6. Bob   •  

    The hard accel is becoming a problem with my Link.
    I have a small light sporty, high rev car.
    So the speed increment example you give is just normal for me.
    Pedal is never to the metal, and I’m not accelerating hard.

    Wondering how this could be adjusted. I guess you need specific data from the OEM’s about each make and model of car. There could be hundreds of thousands of combos to identify and take into consideration. A daunting task. Happy with my Beta Link, looking forward to more tweeks and improvements.

  7. Caroline   •  

    Interesting post Ted! We don’t have much detail like this here in the UK but it would benefit as the problems around harsh accelerating are still quite the same which continues to affect our environment too but also causes issues as fuel is easily wasted by doing this too!

  8. Ted Kidd   •  

    Hey Ted,

    I’m not a fan of really broad generalizations: “(spoiler: driving over 70 MPH is really inefficient)”

    I suspect every car has it’s own individual curve. For example, my TDI Sportwagen seems to start it’s precipitous drop at 65, whereas my old e320cdi didn’t hit that until 80+. (I track this stuff fairly closely. I have my mileage, cost, and fill up data going back to 1998 – http://bit.ly/TKvehiclefuel)

    It would be really cool to see this curve for each model so people could better understand where things line up for their car, seems you have the data to do this.

    • Ted Sumers   •     Author

      Hi Ted!

      You’re right– every car has its own curve, and they do vary– i’d encourage you to check out the link in that sentence ;) there’s an image in that blog post that’s exactly what you’re looking for.

      Also, very cool that you’re manually tracking your mileage so carefully. If you’ve got an automatic, you can look into our IFTTT-to-Google-spreadsheet integration to automate a lot of that: http://blog.automatic.com/automatic-ifttt/

  9. h2oyo   •  

    I dont think its is right to call aggressive driving if you accelerate 0 to 7 mph and say I’m driving aggressively. This needs to be looked at through the rest of the gears used. I may have motivated a little quicker in 1st but the top speed wasnt over 45.That isnt being aggressive it called driving. This insurance plug it in and possibly drop your rates wants you to walk. It needs to look at the whole drive not just 1 second. I never put the pedal to the floor not even close. So if I excellerate to get onto the freeway i will get aggressive driving hit. I have to stop and wait to enter the freeway then i’m expected to get up to freeway speeds slowly, come on.

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