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By C. Jake Williams. July 1, 2008
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Problem: You've read my discourse on drafting while driving and you want to improve your gas mileage, but you're having trouble finding a car to tail that moves at your pace.
Solution: You've found it! Read on, Wayne.
You're interested in slip-streaming behind another vehicle to reduce the aerodynamic resistance on your car, but if a car isn't moving at the speed you want to travel, then it won't do you any benefit as far as drafting.
What follows is my method for finding those cars with your target velocity. The mathematics are listed at the end, but first we'll present the step-by-step lesson on transitioning from cruising solo to drafting at your chosen pace.
The first step is to decide upon your favorite speed to cruise. Five miles over the speed limit is where I like to sit. I'm not setting off any pesky police sirens at that speed, but I'm also able to get around Grandma and Grandpa Nearsighted. It's a win-win. My goal is to draft behind another car moving at 70 mph, assuming a 65 speed limit.
If I get up to the full 70, I'm never going to find another car at my speed. Those cars will remain either a mile ahead of me or a mile behind, and my hopes and dreams of drafting will be for naught. So I only accelerate to 65 mph. I'm still travelling at the speed limit, but now another 70 mph-er can catch up to me.
Anytime a car passes me, I count three seconds. From the time our front bumpers are side-by-side, I'm judging the distance the other car puts between us in that time. If everything goes perfectly, the gap will be 22 feet, the distance an object moving at five mph covers in three seconds. Keep in mind the equations for this are at the bottom.
If another driver puts close to 22 feet between our cars in three seconds, he's moving right around five mph faster than me. And since I'm moving at five mph slower than my goal speed, he's a perfect candidate to keep my wallet a little heavier.
Slide behind that beautiful car already three seconds ahead of you and set your cruise control. You're saving up to ten percent on your fuel efficiency just because you had the patience to let another car catch up to you. And don't feel bad, because you're helping the other cat as well. It's a win-win, another one.
That covers the method of selecting a drafting partner, now here's the math. Start with one mile per hour.
1 mph x (5280 feet/mile) / (3600 seconds/hour) = 1.4667 ft/sec
Now multiply by 5.
5 mph = 7.3333 ft/sec
Now consider only the right side, which is to assume a speed of five miles per hour.
1 second = 7.3333 feet (travelled)
Multiply by three.
3 seconds = 22 feet
Very simply, a car moving at 5 mph will travel 22 feet every 3 seconds. And since velocities can be compared relatively, a car moving 5 mph Faster than you will travel 22 feet Farther than you every 3 seconds.
The method above identifies these cars so you can reap the rewards.
You were there.