I have the opposite issue, I ran a tempo run over 4 miles @ "tempo" pace, which I think I could run faster than if I actually realized my true tempo pace during my winter training. That was on the road at 6:30 pace. I ran a tempo pace on the track, granted I did run two workouts that week already, at the same pace when I was running faster mile/two-mile times and I could only run 3 miles before I felt like I was dying.
I would have to wonder what the conditions are that you are running on on these roads, are you running on gravel, are there slightly slick conditions on the road, is there a decent slope on the side of the road that you are running? Also are you using a device that measures your pace in some manner or another? I think it's one thing to feel like you're running super fast on trails where you could be running around at 8:00/mile pace and it could feel like you're running 6:30/mile. It's another thing to feel this way on a normal road compared to a track.