This morning I finally got around to adding more air to my bike tires. Yesterday, my average speed was 10.4 MPH. This morning: 12.5 MPH. I took 4 minutes off of my commute.
I’ve read, of course, that keeping the correct pressure in your tires is important for fuel efficiency. But it’s not the easiest thing to see with a car: you go the same speed & distance, and unless you’re a number cruncher, it’s hard to compare how much gas you buy.
On the other hand, the “fuel” for a bicycle is your own effort, so it’s really easy to feel how much of that you’re using!