The car I’m driving now – a VW Jetta TDI wagon – has great fuel economy. A careful driver can get up to 50mpg. One of its great features is an option to display moment-by-moment fuel consumption. When accelerating from a stop it may be as low as 8mpg. When I lift my foot from the gas pedal and coast on momentum it can go as high as 200mpg. That provides a powerful incentive to drive more economically. But it has also provided an insight into how reducing speed by relatively small amounts can yield surprisingly large fuel savings. And the feature on my GPS which estimates arrival time at a destination I’ve programmed in shows how little time it costs me when I drive with a lighter foot and a much less thirsty engine.
Twice a week I drive from Coronado, where I’ve been staying, to LaJolla Cove, where I swim. The trip is about 18 miles with 14 of that on I-5. The first few trips I drove the interstate part at 70-75 mph. I noticed that my fuel consumption for those trips was 34-36mpg. When I reduced my speed to 60 mph, fuel efficiency improved to 44-45mpg. I reduced speed by about 17% and improved fuel efficiency by 25%. And my trip duration increased by just two minutes — from about 22 minutes to 24 minutes.
The reason why efficiency increases by 50% more than speed decreases is the geometric effect of wind resistance. A small reduction in speed can mean a significant reduction in energy cost.
There’s a corollary here for swimmers — particularly distance swimmers, and this goes double for triathletes. Because the primary cost of swimming is in overcoming water resistance, and resistance goes up far more than speed, it takes a LOT more energy to swim a little faster.
This means that you can save a LOT of energy by moderating your pace a little bit. And the energy you save can help you hold that pace much longer. Or, in a triathlon, it could help you gain a lot more time on the run (or bike) than you give up on the swim, since – unlike swimming – it takes only a LITTLE energy to run a little faster.
And by practicing Total Immersion Swimming, you’ll probably recover the speed you sacrificed in short order.