## Sunday, January 20, 2008

### More on Cars

I am extremely obsessed with figuring out how to get the best gas mileage out of the car I drive, for a variety of reasons.

First, I think about my contributions to global warming every single day as I make my trip to work. (If only they would make a solar-powered car capable of driving at least 15 miles on a trip, I would be all set.) Second, I am an amateur physicist and I want to understand the workings of the vehicle in which I spend more than five hours a week.

But most relevantly, I am a mathematician, and the idea of optimizing gas mileage as opposed to other possible variables in the equation of driving (such as time or number of miles traveled) sounds really fun and challenging to me, because in the car I drive, I can only ascertain this indirectly.

The car has a manual transmission, so I can control which gear it is in. It also has a crude tachometer, which I can read only to the nearest 125 RPM, a speedometer which I can read to the nearest half-mile per hour, and an odometer which measures distance traveled to the nearest tenth of a mile. Can I use only these tools to optimize my gas mileage on my daily commute?

This is what I've been trying to figure out, actually. I wish I had a good answer. The problem is that I am currently lacking a good intuitive understanding of the way the car works. I've been reading up on torque, horsepower, and RPM, and trying to put it all together. I'll let you know when I finally get there.

Kate said...

Riebecca, I found your post accidentally whilst searching the web for a bug fix idea in my Makefile. And I didn't recognize it was you until I saw too many similarities to myself. Another female applied mathematician. . . in TN!? Wait a second . . . How Funny! Anyway - I love your blog. I ALSO try to max my gas mileage. My car has this addicting feature of telling me the immediate and average gas mileage while I'm driving, and I try to beat my record getting to work each day. Of course the poor souls behind me do suffer as I slowly decelerate getting off the highway, but its saves me acceleration beforehand. I'm nuts.
Keep blogging - excellent work.

CCPhysicist said...

Sure you can. It is easier to monitor the data if the car's computer system does it for you, since you have to be careful to fill the tank to the same level every time if you want to measure consumption with your odometer.

Some things are out of your control (aerodynamics and engine size). Others are under your control (tire inflation pressure, driving style).

Now you can't coast downhill in neutral with the engine off (not legal) or drive on overinflated bald tires (not safe if there is any moisture on the road) like they do in extreme cases, but gentle acceleration, lower speed, and not using "engine braking" when you slow down will all help.

Rapid acceleration (when you produce the most power) really eats gas.

Also, all engines have an optimal rpm for efficiency. It is *probably* around the rpm in top gear that corresponds to the speed the EPA uses for mileage tests (55? I don't know) if the engineers designed it that way. Choosing gears that keep you at that rpm will also help.

Tatanus said...

You could always buy a product that monitors all of your driving habits as well as the engine performance. These devices can record the measurements at a much higher detail than you can with your in-dash instrument panel.
Here is a link to one such product.
It is easy to install and use. There are others, but this one is fairly nice.

Anonymous said...

This is interesting. I am intrigued by the challenge of trying to figure this out yourself rather than just having a car with an electronic dash that includes gas mileage. I hope to hear when you've solved it!