Speed Does Matter

By: Eric
Published On: 5/30/2007 4:15:53 PM

In one the most obvious conclusions in a long time, I'd like to present the findings of my research into gasoline consumption and highway speeds.

Ok, it wasn't exactly scientific research and certainly wasn't a controlled experiment.  It was just an average joe (me) tracking my gasoline consumption vs. speed while on a holiday trip. 

The "duh" conclusion: speed does matter - the faster you go the more gasoline you use. 

But how much?  Read on for the details...
My driving distance was 193 miles each way.  On the way out I forced myself to keep the speed around 65 mph or below, which was the speed limit for much of the journey.  On the way back my average speed was about 6 mph faster.  I'm going to leave out exact speed details of the return trip just in case any state troopers are reading.  ;-)

How did the two compare?

Driving Time
  Out: 3 hours
  Back: 2 hours 45 minutes

Gasoline Used
  Out: 5.8 gallons
  Back: 6.3 gallons

Miles per Gallon
  Out: 33.3
  Back: 30.6

Total Cost (@ Va price of 3.38/gallon)
  Out: $19.60
  Back : $21.29

Cost per mile:
  Out: 10.1 cents
  Back: 11.0 cents

All of this works out to roughly a 10% difference in cost, consumption, and time by driving a moderately slower/faster pace.  On one hand that's not a lot.  An extra $2 in cost when I'm already paying $20 isn't much at all.  Then again, 15 minutes on three hours of travel isn't that much of a time savings either.

I saved a 1/2 gallon of gas (and a little less than $2) going one way and 15 minutes the other. 

If you run the calculations with gasoline at $8 per gallon the extra cost would have been $4 instead of just under $2.  Which is still virtually meaningless financially for a short drive - although it would add up if a driver were to slow down for a whole year.  But be honest, do you really think long term when you're driving down the highway?  I don't.

So, would slowing down help our national gasoline consumption problem?  Absolutely - but only if an overwhelming majority of drivers choose to do so.  My half gallon savings for the holiday weekend didn't do squat at the national level.  Sadly, like other solutions (or improvements) to our oil addiction, I just don't see the vast majority of drivers slowing down. 

Comments



I agree with you (Matt H - 5/30/2007 4:29:58 PM)
But if you were heading out to the hills (either North to PA, or West to WV), you'd use more gas going up hills than down hills.


Yeah - this experiment (Eric - 5/30/2007 4:47:07 PM)
certainly isn't precise, but I think the general indicators are reasonably accurate. 

A more interesting experiment for our commuter community - how does a higher speed compare to a lower speed compare to a traffic jam?  My regular commute takes me out of this one but I'm sure there are many of you who are stuck in gridlock during rush hour and can also travel the same roads when they're clear.  If anyone is up for it I'd love to hear how these compare.



but (blue south - 5/30/2007 4:43:06 PM)
what if you are being paid more than 8 dollars an hour?  Then the extra 15 minutes is worth it.


True. (Eric - 5/30/2007 4:49:33 PM)
It was an off day so my hourly rate didn't factor into the equation. 

Then again, if I had to drive 190 miles each way to commute I'd be looking for a new job.



This was also a factor (littlepunk - 5/30/2007 4:53:36 PM)
Last weekend in the NASCAR race on Sunday night - the frontrunners were all running out of gas with a couple laps to go, and the leader was told to drastically cut back on his speed since he had such a large lead but only a lap to go and was in danger of runnig out of gas.

Bottom line - definitely, your experiment is correct.



True. (Eric - 5/30/2007 6:53:58 PM)
And those NASCAR guys get to do something that I wasn't willing to try - drafting.  I'll bet if I tucked in 6 inches behind a big rig the whole way I'd save a ton of gas.


Not Exactly.... (BobSmith - 5/30/2007 6:33:11 PM)
Eric:

Fuel economy in a vehicle is dependent on a variety of factors, including:

1. Grade of the roadbed
2. Ambient temperature (up to 15% change in fuel economy)
3. Wind speed and direction
4. Altitude

The bottom line is that the only way you could honestly say that an average speed of 6 mph faster had an impact on your fuel economy is to ensure that all these variables (and a few others) were the same.

In addition, the gearing of a vehicle can have a large impact on fuel economy.  Many manufacturers have begun gearing the transmissions on vehicles to take advantage of higher speed limits.  In that case, traveling more slowly in a top gear will, at some point, have an adverse impact on your fuel economy.  This depends on the type of vehicle you are driving.

Before saying you have proved your case, you first need to look at the science behind fuel economy to see where your experiment may have failed.  At a minimum, you should have ensured that you were traveling in the same direction and that weather conditions were roughly similar.  Without that, your change in fuel economy could be the result of extraneous factors unrelated to your higher speed.



Partly agree. (Lowell - 5/30/2007 7:37:19 PM)
I often get different mileage going TO my destination compared to coming BACK.  For instance, when I used to drive to my in laws in West Virginia, before they moved, the trip there was mainly uphill so the mileage was bad.  Coming home, it was almost all downhill so the mileage was much better.  I've also gotten very different mileage readings depending on the weather, temperature, etc.  There are a lot of variables.  Still, all else being equal, driving slower will result in better gas mileage...