Thursday, October 2, 2008

Speed matters!

For the sake of science, I've been making a nuisance of myself over the past month and a half. How so? By sometimes driving exactly speed limit... aside from the occasional Prius, I felt like I was about to be run into more often than not. Why was I doing this? Read on.

Common sense dictates that driving slower uses less gas! It's simple logic... if your engine turns less RPMs, then you are therefore pumping less fuel through it and creating less power.

But, exactly how much does slowing down save? (Or conversely, how much does speeding up cost?)

Over the last little while I've been keeping track of how my speed affects my fuel economy. Using a Scan Gauge (an essential tool for all hyper-milers and anyone concerned with saving gas) I've been able to verify just how much (or how little) fuel I use during my daily commute. Unfortunately, there's far too many variables involved in my entire commute to give anything close to accurate numbers for comparison.

However, there's one stretch of exactly 16.0 kms which is generally congestion free. Many mornings I can turn on the cruise control and not have to touch a pedal for the duration. This doesn't always work though, and from time to time my test was binned because I had to either slow down or speed up to avoid getting into, or causing, accidents.

This stretch includes a few curves in the highway, 3 underpasses, 2 overpasses, one long uphill, and another downhill section.

I ran many tests at varying speeds, alternating randomly between 100kph (the posted speed limit), 105kph, 110kph, and 115kph. The speed driven on any particular day was chosen randomly. The randomness was to prevent any one factor from unduly influencing the results. Things like aerodynamic drag (tried not to spend too much time behind any other cars on the highway), temperature (it was marginally warmer when I started running these tests), the amount of gas in the tank (full vs. empty, different weight), etc. were hopefully negated by averaging out multiple tests over nearly two months of trials. Results that were inexplicably high or low (severe headwind/tailwind perhaps) were thrown out.

Below are the results of 5 different trips at each speed, taken on entirely random days over the last 6+ weeks.

100 kph:
6.8L/100km - 6.5L/100km - 6.4L/100km -6.4L/100km - 6.5L/100km.
Average: 6.52L/100km

105 kph:
6.8L/100km - 6.7L/100km - 7.2L/100km - 7.0L/100km - 6.8L/100km
Average: 6.9L/100km

110 kph:
7.3L/100km - 7.2L/100km - 7.6L/100km - 7.2L/100km - 7.4L/100km
Average: 7.34L/100km

115 kph:
7.8L/100km - 7.7L/100km - 8.1L/100km - 7.9L/100km - 7.7L/100km
Average: 7.84L/100km

So what does speed cost?

Driving 5% faster than the speed limit costs 5.9% more in fuel.

Driving 10% faster than the speed limit costs 12.6% more in fuel.

And finally, driving 15% faster than the speed limit costs you 20.25% more in fuel!

I'd like to try higher speeds to see exactly how much fuel consumption increases with every 5kph, but there are consistantly police along this particular stretch of highway as well, in at least four different spots, so going any faster for the sake of this blog just isn't worth it.

The increase is definitely steep though. Aerodynamic drag increases exponentially with speed, and therefore increases fuel consumption sharply. Looking at my numbers above you can do the math and see that 20% faster than the 100kph speed limit will cost roughly 28-29% more in fuel.

So, is driving fast worth it to you? I suppose it depends how late you are.

Or how much money you want to burn.

No comments: