I've been running on trails for almost five years now. On my high school cross country team, we had a historic rule of thumb: Running at X-minutes per mile pace on trails requires the same amount of energy as running at (X-1)-minute pace on roads, for most reasonable paces (6:00-9:00 pace, in general).
I've read quite a few articles, and all seem to support the idea that trail running requires more energy. It makes sense; uneven surfaces are a lot more challenging to run on than flatter, even surfaces. However, I haven't found anything to support our rule of thumb - that is, that the difference in energy usage is so drastic. Have studies shown that running on trails consumes as much energy as running a minute faster per mile on roads, or is the difference less drastic?
My primary motivation for asking is that I've continued to run trails in college; as in high school, I run in a relatively hilly area. We run quite a bit more on roads than I did in high school, and so as I run higher mileage, I'm trying to figure out if I should run this much slower on trails in order to maintain a constant of effort in my training.