Bluffing the Heart Rate

I can't seem to get enough of riding the single speed on the High Drive trails lately. I have this perfect 1 hour, 10 mile loop that I've settled into over the last month or so. It's got just under 1000' of climbing and lots of swoopy single track. It's a perfect lunch ride.

I track my rides with a Garmin device and I'm noticing that my average heart rate is dropping over time, though my speed is about the same. I'm around 135 average heart rate for these rides. I think this means I'm getting more efficient. And/or maybe there's something going on with the way I'm forced to grind all climbs on the single speed -- all muscle and not as much heart required?

Until I got this single speed, which used to be my CX bike until Glen made me a newer awesomer CX bike, I was riding the Bluff trails on my mountain bike. I spent a few weeks in February trying to see how much elevation I could get out of a single ride on the trails in under 2 hours. I think the best I did was about 2200 feet. On those rides I was averaging about 150 BPM heart rate.

What's it all mean? I feel like I would get better workouts with that higher heart rate. But I'm enjoying the grind of the Bluff on the singlespeed -- I tell myself I'm building muscle or maybe power? I don't care that much -- because I'm enjoying my rides now, so who cares. But I'm curious about what it all means. I'd like my riding to be improving something. It's fun and that's enough, but I'll take improvements too.

Speaking of improvements. I'm going to make some to the single speed. I want to get it all light and rad and I want to go tubeless. As the Bluff trails have gotten more traffic over the years, a bunch of previously, mostly submerged sharp rocks have been exposed. On a mountain bike you can bomb and be ok. But on a bike with 35mm tires, you gotta run about 55psi to keep from getting pinch flats. That's too hard for finessing some of the most fun sections of trail down there. So tubeless will be happening.

For tubeless I need new rims. I'm also going to put a carbon fork and other carbon bits on this sucker since I plan to race it in the fall and since spending a bunch of money on carbon bits will absolutely guarantee it will go twice as fast, at least.