Thursday, April 27, 2006

Cost-Benefit analysis

In fitting with my "hottie nerd" day today, I did a little math regarding gas prices and fuel efficiency. I keep seeing those "Slower Speeds Use Less Gas" signs on the highway, and was wondering if I was coming out ahead or not in terms of fuel consumption versus time. 60% of my commute is at highway speeds (7-8mph over); I never get over 55mph on the back roads or in the city. The internet tells me that for every 5mph that I exceed 55mph, I'm decreasing my fuel efficiency by 5%. So, let's see what happens.

Current price of gas = $3.06, current mpg = 30.
[insert a page of math here that I won't bore you with]
It costs me an extra 35.8 cents per trip to drive at my current speed than at 55mph. But I save 6.42 minutes per trip. Hopkins tells me that, as a grad student, my time is worth $12.81 an hour, or 21.4 cents per minute. So I'm "saving" $1.37 per trip, with a net "savings" of $2.02 per day.

Gas would have to more than triple in price before slowing down would make financial sense over the time savings. And if gas triples, I've got bigger problems than a reduction in fuel efficiency.

Oh, also while I was driving today, I saw a deaf guy riding in a car and signing to the driver (I mean to say, I assume he was deaf because he was signing to the driver). What I want to know is how she could possibly pay attention to the road and to what he was saying. That seems like an accident waiting to happen.

13 comments:

Edge said...

I think ethanol may be the only solution so far if our farmers can sustain the demand. Nice math BTW.

~Jef

Courtney said...

Ethanol's got its own set of problems (consumes more energy to produce than it supplies) but I'm all for paying less at the pump. I figure by the time I'm ready to buy my next car, the hybrids will be much better and not butt-ugly. I have high hopes for the Hybrid Accord; I'm not a horsepower freak but the idea of left-merging onto the interstate in a 95HP Hybrid Civic scares me a little.

goodbye said...

I left-merge onto I-95 in my 92HP Mitsubishi Mirage every single day. It does just fine.

Courtney said...

Good for you :-) It's not my bag of chips.

Staci said...

I would rather claw my eyes out than calculate how much money I would save from reducing my speed by 5 mph. It kind of amazes me what you choose to do sometimes.

Courtney said...

Amazement goes both ways, dear.

eric said...

i don't trust that you save that much fuel per 5 mph over 55. not as a blanket statement for every car, anyway. it depends on the car's gear ratios, whether the driver is accelerating or just holding speed, what gear you're in, etc.

so, beyond your calculations that prove their claims worthless, there are other reason i think it's worthless to ignore those signs.

Courtney said...

Yeah, I know that too. I just used that as my "estimate." Plus, I bet the state spends more money in energy keeping those signs lit all day and night than I could save by slowing down 5mph.

Kim, where are you left-merging anyways? It was my understanding that the only left-merge on that stretch of I-95 was at rt. 32.

Staci said...

I know, you don't get me either.

Out of curiosity, how much did Hopkins pay you to do your calculations? :)

Courtney said...

$1.07

goodbye said...

I took 'left-merge' to mean merge left onto 95. I have, however, merged onto I-95 from the left MANY times. I had to do it everyday while working in Tysons Corner. The ramp to I-95 from 495 comes up on the left. I also get on 95 from Rte. 32 sometimes as well.

Even still, I merge onto 95 at exit 47 and managed to go fast enough to get into the left lane before exit 49 - all on 92HP. My car only starts to shake when I start going around/over 80mph. ;)

Jim said...

Unfortunately, the Accord Hybrid is more about using hybrid technology to gain horsepower than it is to save fuel. At least the V6 hybrid is.

As for the power of cars like the Prius or Hybrid Civic, they are easily comprable to cars like the gas civic, focus, etc. You have to remember, HP isn't the whole story, torque plays in a lot too, especially in low speed pickup (eg, the first half or so of a 0-60 time). And electric engines produce huge torque - way more than a gas engine. Its easy to squeal the tires of a hybrid, or so I hear. In addition, things like the continuously variable transmission can serve to maximize the power output of the smaller engine.

Before you dismiss hybrids you should drive one - I think you'll find they are every bit as quick (or close to it) as your focus or Matt's cavalier.

Of course, the other problem is bottom line wise, its still cheaper to own something like a Toyota Echo, because gas prices aren't high enough to outweigh the increased cost of the hybrid vehicle.

Courtney said...

I agree with a lot of what you've said. But I'm not going to be in the market for another new car for at least 4 more years. Who knows how much gas will cost then, and I'm sure the hybrid technology will be better and cheaper as well. I also don't want to compromise safety for a few dollars at the pump; the Toyota Echo looks like it would be completely squashed if it hit anything.