The three-second rule is a good one for safety, flexibility and legality. It's easy to employ without having to take your eyes off the road. We do it in my area of the world by saying "one Mississippi" for each second of time being gauged.
I used to use a vacuum gauge installed on the intake manifold to see the beneficial effects of following a big truck etc. Nowadays, I have a ScanGauge with instantaneous MPG readings to do the same.
One of the big things I keep in mind when following a vehicle in traffic is that rear-end accidents can be expensive and even lethal, and that the guy who does the rear-ending is almost invariably at fault.
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