Ruckstell axle...underdrive or overdrive?

[stickshift]

I've always thought the Ruckstell axle was an under drive. Most of my driving is with the shifter pulled back, or in low. Last weekend I drove my Model T into town delivering gifts to relatives so they could also see my "T" that they heard about, and on the way home I decided to get on the highway and see what she's got. I was topped out at 45 and put the shifter forward. The engine actually lugged until I gained more speed. I was going over 50, which I didn't really want to do, but like I said the engine needed more RPM. I think I could have easily gone 55 but I slowed down and pulled the Ruckstell shifter back again to stay at 45. This tells me that it's an overdrive, right?