It is NTSC or 480i Upconverted to 720p. Unless the definition has changed it is SD upconverted.
SpeedHD is not SD.
This is due to DirecTV rez-ing channel 607 to 480x480 - i.e., lower resolution then SD. And the pixelization is usually due to DIrecTV taking bandwidth from channel 607.
Anyone that compares the 2 will see that normal Speed is way more pixelated than SpeedHD (even in 4:3 aspect).
It is probably actual SD instead of the down rez'ing done on 607. So how good it looks depends on how good upconverted SD looks on your TV.
So while it is 4:3, SpeedHD is much sharper. It will make a bigger difference, the bigger your screen is.