Monday, May 19, 2008

Driving 60 MPH Improves Mileage (but Staying Home Uses Even Less Gas!)

Last week I drove round trip to pick up my college student daughter. I rarely drive long distances; mostly I do short urban runs which get terrible mileage. My Subaru Forester can only go about 250 miles a tank for local driving. I watch the tripometer pretty closely because I've run out of gas a few times - the gas gauge was defective. It's been replaced, but I still pay attention.
I drove 60 MPH consistently, and used the strategies you read about to improve mileage - take your foot off the gas and coast, instead of breaking to slow down, that sort of thing. When I arrived at her school, 200 miles from my house, I was astonished when the gauge measured only half empty. That is practically double my normal mpg. On the return trip, to my amazement, we just kept driving on that tank of gas. First we crossed 300 on the tripometer. I've done that before on highways, so I wasn't that surprised. Then 350. Then close to 400. The needle kept going down, but by then I was getting really nervous that it was broken. But being on such a roll, I also wanted to see just how far I could go on that damn tank of gas! Then it was about 30 miles before a visible gas station appeared. When I filled my tank, there was still about 1/8 tank left - the light hadn't even come on! Driving 60 mph improved my mpg by about 30% at least.
This is only good news if you might otherwise drive 65 or 70, of course. The best way to save gas is - DO NOT DRIVE.


Coach Parker said...

This isn't terribly surprising. At highway speeds, wind resistance is your biggest source of friction. (If it weren't for friction, it would take no gas to travel, as inertia would carry you)

Wind resistance is proportional to the square of your speed. So at 60mph you get almost twice the mpg as at 85mph-- well, not quite that because of acceleration cost and the engine's internal friction, but it's pretty close to that.

Wind resistance is the reason nowadays it's suggested that you use the A/C instead of rolling down the windows at highway speeds-- of course, you get better gas mileage with neither, but the A/C is efficient enough to be better on the gas tank than open windows.

Heather said...

My grandpa always told me that I should take my car out on the highway once a week and get up to those speeds for a little bit. I'm no mechanic, but it's supposed to help keep your system flushed out and keep your milage higher than normal even when you're in-city. Of course it's been years since I've owned a car, but he lived by it.