double nickels

If we are really concerned about saving money on oil, conservation is the only short term path. There are many tactics that drop miles driven and we have seen two and three percent drops in recent months (year to year).

But enforcing an efficient speed limit is something that is viewed as political suicide. Senator Warner has called for looking at the idea of a 55 mph national speed limit, but he is only calling for studies and no one is joining him.

There have been a few studies on the effectiveness of the 55 mph speed limit when it was Federal law (one is here). Most of the studies show 2,000 to 4,000 fewer deaths per year and a 2% drop in oil consumption. They also note that compliance was only about 25% to 30%.

Finding the optimal mileage range of a car is tricky as there are a variety of components. Some have no velocity dependence (running your A/C and electricals) and others that increase linearly, as the square and as the cube of velocity. Above 40 mph, wind resistance begins to dominate. The power to overcome it goes as the cube of velocity. The shape of a car influences its coefficient of drag (called the Cd) and some vehicles are much "slipperier" than others. A gas or diesel engine has a fairly narrow power range and a transmission is used to match this efficient range with the speed of the car. How well a car is geared and shifted moves the sweet spot for best economy. Most manufacturers, worried about CAFE fines, optimize for 50-55 mph. It is possible to move a bit higher - 60 or even 65, but that demands a low Cd and the mileage will not be as good as if the manufacturer had re-geared the car (not only Ferrari, but some high end German makes are famous for optimizing a bit on the high side - they could improve their fuel scores significantly by re-gearing).

In the past few weeks I've exchanged mail with an engineer who design cars for a living (a German company). His view is that you can't fool physics. With drag coefficients that we see on normal cars an enforced 55 mph speed limit would drop fuel consumption dramatically over what we see on highways today. He said people talk about 6 to 9 percent in the US with the current speed and vehicle mix (this may be very consistent with the observations of the old speed limit). If there was an enforced national speed limit, you would probably also see manufacturers re-gearing their offerings.

Of course people wouldn't get to where they are going as quickly and there may or may not be economic impacts from this, but if you want to save oil a strictly enforced national speed limit is probably the quickest path. A few people (say those with high end touring cars) will see worse mileage, but the vast majority will see an improvement.

I doubt it will happen.

One thing you can do is clean up you vehicle. Things that protrude can dramatically increase the Cd of your car. Roof racks are bad, a closed tailgate on an open-bed pickup can cost a few mpg at 60 mph. Even the rough surface on a dirty car can make a difference (this is why you wash airplanes - it is very cost effective).

What I don't recommend is driving 55 when the traffic is flowing at 70 - difference in speed can be a dangerous thing.