Originally Posted by chadta
brakes are cheaper than engines and clutches
With proper engine braking the clutch is fully engaged and the engine is not under load because there is no combustion (see below) going on in the engine, which is means there's no power stroke (the part of the cycle that causes the most wear), and thus less wear than when under power.
In most cases with a modern automatic transmission vehicle the only thing required to maintain a controlled highway speed is to turn off the overdrive and take your foot off the throttle. When driving in mountainous terrain you should have your overdrive off anyway as it will constantly be engaging and disengaging (causing a lot of excess wear) if you don't.
With a standard transmission, engine breaking amounts to shifting twice to travel 5 miles down a hill vs shifting dozens of times to travel the same distance in a city. On that topic, just driving around the city puts far more wear on your vehicle than anything you might do on the freeway. It's not even really a comparison as far as wear goes.
When you factor in that you have a lot
more control over the vehicle, it's a no brainer.
It doesn't even use more gas in an fuel injected engine (almost any vehicle built since the early 90s). The ECU of any electronic fuel injected engine has a throttle sensor which will stop injecting fuel (unless at idle) if there is no throttle. Even mechanical fuel injectors had similar functionality.