Ok, actually around a year and a half. I drive a Honda Insight Hybrid for my commute, which for part of this time was 40.6 miles round trip, and 38.6 miles for the rest of that time. The Honda insight has great mileage, and the instrumentation makes it very easy to record both daily mileage as well as overall lifetime mileage. Once I learned how easy it was to reset the mileage calculation daily ( or hourly if you really wanted to) I thought it would be interesting to learn what cycles or patterns might show up to cause the overall average to be what it was.
I did not turn in to a hypermilage practitioner, but I did learn some things about the car and my driving patterns. Overall, I think it improved my mileage, since I did alter my habits a bit.
June 29- 2010: 43 miles, and 59.4 MPG
Dec. 22-2011: 46.5 miles, 50 MPG
These two entries point out one of the most significant findings. I thought I had noticed a seasonal swing in mileage before, but now it was quite evident, and I looked in to it and found three main contributors to this pattern. First, the engine does not reach full efficiency till it is warmed up above a certain temperature. I'm not an expert in ICEs, or even a car nut, but posts on Hybrid owner forums corroborated this. Second, the computer in the car had been programmed to keep the engine running at stops if the defrost was turned on. I presume this was a design decision that said visibility and safety trump mileage. Third, I live in an EPA ozone nonatainment area, meaning gasoline formulation changes between winter and summer. I have no idea how the Toyota engineers dealt with this, but it would be interesting to compare notes with a Prius owner. Anyway, just knowing these things changed my habits a bit. After my windows were clear, I was much more attentive to turn off the defrost until needed. One of the huge ways hybrids save gas is by shutting off the engine at stops, and if you take away that trick, mileage plummets. If all cars had instantaneous mileage gauges, and could easily track this, commuters would be up in arms demanding hybrids and better stop light algorithms.
I've never counted, but I would estimate my commute has around 30stop lights, and let's just say if I was an Xmen mutant, my special mutation was causing lights to turn red. Anyway, I'll cover other things I learned in future posts.